xfs: don't return garbage buffers in xfs_da3_node_read

If we're reading a node in a dir/attr btree and the buffer comes off the
disk with a magic number we don't recognize, don't ASSERT and don't set
a garbage buffer type (0 also triggers ASSERTs).  Instead, report the
corruption, release the buffer, and return -EFSCORRUPTED because that's
what the dabtree is -- corrupt.

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