fs: add two type safe mapping helpers

Introduce i_{g,u}id_into_vfs{g,u}id(). They return vfs{g,u}id_t. This
makes it way harder to confused idmapped mount {g,u}ids with filesystem
{g,u}ids.

The two helpers will eventually replace the old non type safe
i_{g,u}id_into_mnt() helpers once we finished converting all places. Add
a comment noting that they will be removed in the future.

All new helpers are nops on non-idmapped mounts.

Link: https://lore.kernel.org/r/20220621141454.2914719-3-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>