xfs: use a local variable for magic number in xfs_da3_node_lookup_int

Use a local variable for the block magic number checks instead of
abusing blk->magic.

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