xfs: fail if xattr inactivation hits a hole

The child buffer read in xfs_attr3_node_inactive() should never
reach a hole in the attr fork. If this occurs, it is likely due to a
bug. Prior to commit cd87d867 ("xfs: don't crash on unexpected holes
in dir/attr btrees"), this would result in a crash. Now that the
crash has been fixed, this is a silent failure.

Pass -1 to xfs_da3_node_read() from xfs_da3_node_inactive() to
indicate that reading from a hole is an error. This logs an error to
syslog and fails the inode inactivation, leaving the inode on the AG
unlinked list until removed by xfs_repair (or log recovery). Also
update the subsequent code to reflect that the read now returns a
non-NULL buffer or an error.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>