f2fs: skip committing valid superblock

In recovery procedure for superblock, we try to write data of valid
superblock into invalid one for recovery, work should be finished here,
but then still we will write the valid one with its original data.
This operation is not needed. Let's skip doing this unnecessary work.

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