f2fs: fix comment in sanity_check_raw_super()

Commit d7e9a903

 ("f2fs: Support Block Size == Page Size") missed to
adjust comment in sanity_check_raw_super(), fix it.

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>