linuxkpi: Define struct vfsmount in <linux/mount.h>

In the context of the DRM drivers, this is used to show GEM objects in a
shmfs virtual filesystem. The new shmem_file_setup_with_mnt() - also
introduced in this commit as an alias to shmem_file_setup() - takes a
struct vfsmount as its first argument to indicate which shmfs mount
should be used.

For now, the structure is empty. As we don't present GEM objects in a
virtual filesystem right now, we can defer the actual implementation of
this structure once we have an actual use for it.

The DRM generic code started to use it in Linux 6.13.

Reviewed by: emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D57572