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
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>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/acl.c

@@ -98,7 +98,7 @@ int fuse_set_acl(struct user_namespace *mnt_userns, struct inode *inode,
 			return ret;
 		}
 
-		if (!in_group_p(i_gid_into_mnt(&init_user_ns, inode)) &&
+		if (!vfsgid_in_group_p(i_gid_into_vfsgid(&init_user_ns, inode)) &&
 		    !capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_FSETID))
 			extra_flags |= FUSE_SETXATTR_ACL_KILL_SGID;