md/bitmap: revert a patch

This reverts commit 8031c3dd. That patches doesn't work well if PAGE_SIZE >
4k. We will fix the original problem with a different approach.

Fix: 8031c3dd(md/bitmap: copy correct data for bitmap super)
Reported-by: Joshua Kinard <kumba@gentoo.org>
Cc: stable@vger.kernel.org (4.10+)
Suggested-by: Neil Brown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>