fuse: warn if fuse_access is called when idmapped mounts are allowed

It is not possible with the current fuse code, but let's protect ourselves
from regressions in the future.
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/dir.c

@@ -1474,6 +1474,14 @@ static int fuse_access(struct inode *inode, int mask)
 
 	BUG_ON(mask & MAY_NOT_BLOCK);
 
+	/*
+	 * We should not send FUSE_ACCESS to the userspace
+	 * when idmapped mounts are enabled as for this case
+	 * we have fc->default_permissions = 1 and access
+	 * permission checks are done on the kernel side.
+	 */
+	WARN_ON_ONCE(!(fm->sb->s_iflags & SB_I_NOIDMAP));
+
 	if (fm->fc->no_access)
 		return 0;