xfs: only check da node header padding on v5 filesystems

It turns out that we only started zeroing a new da btree node's block
header on v5 filesystems.  Prior to that, we just wouldn't set anything
at all, which means that the pad field never got set and would retain
whatever happened to be in memory.

Therefore, we can only check the pad for zeroness on v5 filesystems.
shared/006 on a v4 filesystem exposes this scrub bug.

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