btrfs: add_missing_dev() should return the actual error

add_missing_dev() can return device pointer so that IS_ERR/PTR_ERR can
be used to check for the actual error that occurred in the function.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
[ minor error message adjustment ]
Signed-off-by: David Sterba <dsterba@suse.com>