f2fs: fix recording invalid last_victim

When doing garbage collection, we try to record segment offset which
locates at next one of last victim, using it as the start offset in
next searching.

But in some corner cases, recorded offset may cross the end of main
segment area, it will cause incorrectly searching in dirty_segmap
bitmap. This patch adds modular operation to avoid this issue.

Reported-by: Yunlei He <heyunlei@huawei.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>