xfs: btree lookup shouldn't ASSERT on empty btree nodes

xfs: btree lookup shouldn't ASSERT on empty btree nodes

Source kernel commit: eeee0d6a9bc93eaa211918c203fde263d44fa20e

If a btree lookup encounters an empty btree node or an empty btree leaf
on a multi-level btree, that's evidence of a corrupt on-disk btree.
Therefore, we should return -EFSCORRUPTED to the upper levels, not an
ASSERT failure.

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