nilfs2: initialize unused bytes in segment summary blocks

nilfs2: initialize unused bytes in segment summary blocks

Syzbot still reports uninit-value in nilfs_add_checksums_on_logs() for
KMSAN enabled kernels after applying commit 7397031622e0 ("nilfs2:
initialize "struct nilfs_binfo_dat"->bi_pad field").

This is because the unused bytes at the end of each block in segment
summaries are not initialized.  So this fixes the issue by padding the
unused bytes with null bytes.

Link: https://lkml.kernel.org/r/20230417173513.12598-1-konishi.ryusuke@gmail.com
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Tested-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Reported-by: syzbot+048585f3f4227bb2b49b@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?extid=048585f3f4227bb2b49b
Cc: Alexander Potapenko <glider@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>