fs/mnt_idmapping.c: Return -EINVAL when no map is written

Currently, it is possible to create an idmapped mount using a user
namespace without any mappings. However, this yields an idmapped
mount that doesn't actually map the ids. With the following change,
it will no longer be possible to create an idmapped mount when using
a user namespace with no mappings, and will instead return EINVAL,
an “invalid argument” error code.

Reviewed-by: Christian Brauner <[e-mail]>
Signed-off-by: Taylor Jackson <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Christian Brauner <[e-mail]>

fs/mnt_idmapping.c

···
 	 * anything at all.
 	 */
 	if (nr_extents == 0)
-		return 0;
+		return -EINVAL;
 
 	/*
 	 * Here we know that nr_extents is greater than zero which means