nfs4: minor improvements in VnodeToInode code * removal of now unnecessary NULL check * Clear() is Replace(NULL)

nfs4: support hard links properly

The main purpose of this patch is to prevent VFS from removing a vnode to early
what might have happened if the NFS client knew it had more than one name but
then one of them was deleted. Moreover, all discovered and still valid names
are stored what may be useful in proper file handle recovery when they are
volatile.

This patch fixes both #9558 and #9561.

nfs4: Do not remove RootInode too soon

nfs4: Do not create new Inode object when deleting a vnode

nfs4: Introduce VnodeToInode mapping

VnodeToInode is another abstraction layer between VFS vnodes and NFS nodes.
It simplifies dealing with race conditions as well as faking some guarantees
VFS expects but NFS does not provide.

nfs4: minor improvements in VnodeToInode code * removal of now unnecessary NULL check * Clear() is Replace(NULL)

nfs4: support hard links properly

The main purpose of this patch is to prevent VFS from removing a vnode to early
what might have happened if the NFS client knew it had more than one name but
then one of them was deleted. Moreover, all discovered and still valid names
are stored what may be useful in proper file handle recovery when they are
volatile.

This patch fixes both #9558 and #9561.

nfs4: Do not remove RootInode too soon

nfs4: Do not create new Inode object when deleting a vnode

nfs4: Introduce VnodeToInode mapping

VnodeToInode is another abstraction layer between VFS vnodes and NFS nodes.
It simplifies dealing with race conditions as well as faking some guarantees
VFS expects but NFS does not provide.