mnt_idmapping: add vfs{g,u}id_t

mnt_idmapping: add vfs{g,u}id_t

Introduces new vfs{g,u}id_t types. Similar to k{g,u}id_t the new types
are just simple wrapper structs around regular {g,u}id_t types.

They allows to establish a type safety boundary between {g,u}ids on
idmapped mounts and {g,u}ids as they are represented in filesystems
themselves.

A vfs{g,u}id_t is always created from a k{g,u}id_t, never directly from
a {g,u}id_t as idmapped mounts remap a given {g,u}id according to the
mount's idmapping. This is expressed in the VFS{G,U}IDT_INIT() macros.

A vfs{g,u}id_t may be used as a k{g,u}id_t via AS_K{G,U}IDT(). This
often happens when we need to check whether a {g,u}id mapped according
to an idmapped mount is identical to a given k{g,u}id_t. For an example,
see vfsgid_in_group_p() which determines whether the value of vfsgid_t
matches the value of any of the caller's groups. Similar logic is
expressed in the k{g,u}id_eq_vfs{g,u}id().

The from_vfs{g,u}id() helpers map a given vfs{g,u}id_t from the mount's
idmapping into the filesystem idmapping. They make it possible to update
a filesystem object such as inode->i_{g,u}id with the correct value.

This makes it harder to accidently write a wrong {g,u}id anwywhere. The
vfs{g,u}id_has_fsmapping() helpers check whether a given vfs{g,u}id_t
can be mapped into the filesystem idmapping.

All new helpers are nops on non-idmapped mounts.

I've done work on this roughly 7 months ago but dropped it to focus on
the testsuite. Linus brought this up independently just last week and
it's time to move this along (see [1]).

[1]: https://lore.kernel.org/lkml/CAHk-=win6+ahs1EwLkcq8apqLi_1wXFWbrPf340zYEhObpz4jA@mail.gmail.com

Link: https://lore.kernel.org/r/20220621141454.2914719-2-brauner@kernel.org
Cc: Seth Forshee <sforshee@digitalocean.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
CC: linux-fsdevel@vger.kernel.org
Reviewed-by: Seth Forshee <sforshee@digitalocean.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>