f2fs: refactor next blk selection

Remove __refresh_next_blkoff by opencoding the SSR vs LFS segment check
in the only caller, and then add helpers for SSR block selection and
blkoff randomization instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>