f2fs-tools: remove unnecessary alignment for blocks_for_ssa

total_valid_blks_available is already a multiple of c.blks_per_seg.
So, we don't need an additional alignment adjustment.

Signed-off-by: Daeho Jeong <daehojeong@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>