xfs: check directory data block header padding in scrub

xfs: check directory data block header padding in scrub

Add the missing scrub check for the pad field in directory data block
headers. Old kernels may have written non-zero padding without issue,
and the write path now self-heals stale padding on modification. Flag
non-zero padding as an optimization opportunity (preen) rather than
corruption.

Add xchk_fblock_set_preen helper for reporting file fork block issues
that could be optimized. The trace event xchk_fblock_preen already
exists.

Signed-off-by: Yuto Ohnuki <ytohnuki@amazon.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>