Revert "linuxkpi: `GFP_KERNEL` equals `M_NOWAIT` now"

This change seems to break some drivers such as the mlx5*(4) drivers.

As kib@ says:
> According to the 'official' Linux kernel documentation, the GFP_KERNEL
> flag implies sleepable context.

It was introduced while working on the new vt(4)/DRM integration [1].
During this work, doing sleepable allocations broke vt(4) and the DRM
drivers. However, I made further improvements and some locking-related
fixed to the new integration without revisiting the need for it.

After more testing, the improvements to the integration mentionned above
seems to make the change to `GFP_KERNEL` unneeded now. I can thus
revert it to restore expectations of other drivers.

This reverts commit 14dcd40983748596d116d91acb934a8a95ac76bc.

[1] https://github.com/freebsd/drm-kmod/pull/243

Reviewed by: kib
Approved by: kib
Differential Revision: https://reviews.freebsd.org/D42962

sys: Automated cleanup of cdefs and other formatting

Apply the following automated changes to try to eliminate
no-longer-needed sys/cdefs.h includes as well as now-empty
blank lines in a row.

Remove /^#if.*\n#endif.*\n#include\s+<sys/cdefs.h>.*\n/
Remove /\n+#include\s+<sys/cdefs.h>.*\n+#if.*\n#endif.*\n+/
Remove /\n+#if.*\n#endif.*\n+/
Remove /^#if.*\n#endif.*\n/
Remove /\n+#include\s+<sys/cdefs.h>\n#include\s+<sys/types.h>/
Remove /\n+#include\s+<sys/cdefs.h>\n#include\s+<sys/param.h>/
Remove /\n+#include\s+<sys/cdefs.h>\n#include\s+<sys/capsicum.h>/

Sponsored by: Netflix

linuxkpi: `GFP_KERNEL` equals `M_NOWAIT` now

... instead of `M_WAITOK`.

[Why]
The reason is that in some places in the DRM drivers (in particular, the
framebuffer management code), kmalloc() is called from a non-sleepable
context, such as after a call to mtx_lock(8) with an MTX_DEF mutex.

If `GFP_KERNEL` is defined as `M_WAITOK`, we hit an assertion from
witness(4).

[How]
The definition of `GFP_KERNEL` is changed to `M_NOWAIT`. This means that
callers should verify the return value of kmalloc(). Fortunately, this
is always the case in Linux.

Reviewed by: bz, emaste, manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D42054

sys: Remove $FreeBSD$: one-line .c pattern

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\n/

LinuxKPI: reduce usage of struct vm_page and vm_page_t

We currently define (Linux) page to (FreeBSD) vm_page.
Cleanup some of the direct struct vm_page and vm_page_t declarations
and usages in the Linux KPI and make them 'struct page' or
'struct page *' to prepare for more upcoming work.

This should be a NOP.

Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D41255

vm_object: Modify various drivers to allocate OBJT_SWAP objects

This is in preparation for removal of OBJT_DEFAULT. In particular, it
is now cheap to check whether an OBJT_SWAP object has any swap blocks
allocated, so the benefit of having a separate OBJT_DEFAULT type is
quite marginal, and the OBJT_DEFAULT->SWAP transition is a source of
bugs.

Reviewed by: alc, hselasky, kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35779

linuxkpi: Move shmem related functions in it's own file

For drmkpi (D23085) we don't want the Linux struct file as we don't emulate
everything. Also the prototypes should be in shmem_fs.h to have 100%
compatibility with Linux.

Reviewed by: hselasky
MFC after: Maybe
Differential Revision: https://reviews.freebsd.org/D23764