fat: port to vfs{g,u}id_t and associated helpers

A while ago we introduced a dedicated vfs{g,u}id_t type in commit
1e5267cd ("mnt_idmapping: add vfs{g,u}id_t"). We already switched
over a good part of the VFS. Ultimately we will remove all legacy
idmapped mount helpers that operate only on k{g,u}id_t in favor of the
new type safe helpers that operate on vfs{g,u}id_t.

Cc: Seth Forshee (Digital Ocean) <sforshee@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>