xfs: error out if trying to add attrs and anextents > 0

xfs: error out if trying to add attrs and anextents > 0

Source kernel commit: 0f352f8ee8412bd9d34fb2a6411241da61175c0e

We shouldn't assert if somehow we end up trying to add an attr fork to
an inode that apparently already has attr extents because this is an
indication of on-disk corruption.  Instead, return an error code to
userspace.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>