userns: Check uid_map's opener's fsuid, not the current fsuid

Signed-off-by: Andy Lutomirski <luto@amacapital.net>

kernel/user_namespace.c

@@ -797,12 +797,12 @@ static bool new_idmap_permitted(const struct file *file,
 		u32 id = new_map->extent[0].lower_first;
 		if (cap_setid == CAP_SETUID) {
 			kuid_t uid = make_kuid(ns->parent, id);
-			if (uid_eq(uid, current_fsuid()))
+			if (uid_eq(uid, file->f_cred->fsuid))
 				return true;
 		}
 		else if (cap_setid == CAP_SETGID) {
 			kgid_t gid = make_kgid(ns->parent, id);
-			if (gid_eq(gid, current_fsgid()))
+			if (gid_eq(gid, file->f_cred->fsgid))
 				return true;
 		}
 	}