xfs: zero directory data block padding on write verification

xfs: zero directory data block padding on write verification

Old kernels did not zero the pad field in xfs_dir3_data_hdr when
initializing directory data blocks, so existing filesystems may have
non-zero padding on disk.

Zero the pad field in xfs_dir3_data_write_verify alongside the existing
LSN and checksum updates. The pad field is pure alignment padding with
no runtime meaning, so zeroing it during write verification is safe and
has no additional I/O cost. This lets filesystems gradually self-heal
stale non-zero padding as directories are modified, without requiring an
explicit repair pass.

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