f2fs: fix block migration when section is not aligned to pow2

As for zoned-UFS, f2fs section size is forced to zone size. And zone
size may not aligned to pow2.

Fixes: 859fca6b

 ("f2fs: swap: support migrating swapfile in aligned write mode")
Signed-off-by: Liao Yuanhong <liaoyuanhong@vivo.com>
Signed-off-by: Wu Bo <bo.wu@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>