f2fs: refactor f2fs_commit_super

Previously, f2fs_commit_super hacks the bh->blocknr to write the broken
alternate superblock.
Instead of it, we should use the correct logic to retrieve its buffer head
with locking it appropriately.

Reviewed-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>