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

fuse: 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
1e5267cd0895 ("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: Christoph Hellwig <hch@lst.de>
Reviewed-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>