xfs: check da node magic in _node_lookup_int

Before we start processing what we /think/ is a da3 node block, actually
check the magic to make sure that we're looking at a node block.  This
way we won't blow the asserts in _node_hdr_from_disk on corrupted
metadata.

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