MFC r312432:
Add a mount option for tmpfs(5) to not use namecache.

MFC r312430:
Implement VOP_VPTOCNP() for tmpfs.

MFC r312428:
Refcount tmpfs nodes and mount structures.

MFC r312410:
Rework some tmpfs lock assertions.

MFC r312412:
Protect macro argument.

MFC r312409:
Style fixes and comment updates.

MFC r312435:
Remove mistakenly merged field.

MFC r311525:
Lock tmpfs node tn_status updates done under the shared vnode lock.

MFC r295574:
Clear the cookie pointer on error in tmpfs_readdir().

Approved by: re (glebius)

MFC r277828:
Update mtime for tmpfs files modified through memory mapping.

MFC r277969:
Update both ctime and mtime for writes to tmpfs files.

MFC r277972:
Remove single-use boolean.

MFC r278151:
Remove duplicated assignment.

MFC r276007:
Handle MAKEENTRY cnp flag in the VOP_CREATE().

MFC r275897:
Set NOCACHE flag for CREATE namei() calls, do not specially handle
MAKEENTRY in VOP_LOOKUP().

MFC r268765:
Remove unused header.

MFC r268764:
Check for the cross-device cross-link attempt in the VFS, instead of
VOP_LINK() implemenations.

MFC r268766:
Do not ignore error from tmpfs_alloc_vp().

MFC r268614:
Use tmpfs_vn_get_ino_gen() to handle the races with reclaim in tmpfs
dotdot lookup.

MFC r268613:
Style. Add comment about lock mode.

MFC r268609:
Add some assertions for the code handling vm_object for tmpfs vnode.

MFC r268608:
The tmpfs_link() must not dereference the filesystem-specific data for
a vnode until it is verified that the vnode indeed belongs to tmpfs mount.

MFC r268605:
Remove code separator lines which do not conform to style(9).

MFC r267564:
In msdosfs_setattr(), add a check for result of the utimes(2) permissions test.
Refactor the permission checks for utimes(2).

MFC r263131,r263174,r263175:

Tmpfs readdir() redundant logic and code readability cleanup.

r263131:
Cleanup redundant logic and add some comments to help explain how it works
in lieu of potentially less clear code.

r263174:
Rename cnt to maxcookies and change its use as the condition for when to
lookup cookies to be less obscure.

r263175:
Add missing FALLTHROUGH comment in tmpfs_dir_getdents for looking up '.'
and '..'.

MFC r277828:
Update mtime for tmpfs files modified through memory mapping.

MFC r277969:
Update both ctime and mtime for writes to tmpfs files.

MFC r277972:
Remove single-use boolean.

MFC r278151:
Remove duplicated assignment.

MFC r276007:
Handle MAKEENTRY cnp flag in the VOP_CREATE().

MFC r275897:
Set NOCACHE flag for CREATE namei() calls, do not specially handle
MAKEENTRY in VOP_LOOKUP().

MFC r268765:
Remove unused header.

MFC r268764:
Check for the cross-device cross-link attempt in the VFS, instead of
VOP_LINK() implemenations.

MFC r268766:
Do not ignore error from tmpfs_alloc_vp().

MFC r268614:
Use tmpfs_vn_get_ino_gen() to handle the races with reclaim in tmpfs
dotdot lookup.

MFC r268613:
Style. Add comment about lock mode.

MFC r268609:
Add some assertions for the code handling vm_object for tmpfs vnode.

MFC r268608:
The tmpfs_link() must not dereference the filesystem-specific data for
a vnode until it is verified that the vnode indeed belongs to tmpfs mount.

MFC r268605:
Remove code separator lines which do not conform to style(9).

MFC r267564:
In msdosfs_setattr(), add a check for result of the utimes(2) permissions test.
Refactor the permission checks for utimes(2).

MFC r263131,r263174,r263175:

Tmpfs readdir() redundant logic and code readability cleanup.

r263131:
Cleanup redundant logic and add some comments to help explain how it works
in lieu of potentially less clear code.

r263174:
Rename cnt to maxcookies and change its use as the condition for when to
lookup cookies to be less obscure.

r263175:
Add missing FALLTHROUGH comment in tmpfs_dir_getdents for looking up '.'
and '..'.