f2fs: alloc new section if curseg is not the first seg in its zone

If curseg is not the first segment in its zone, the zone is not empty.
A new section should be allocated and avoid resetting the old zone.

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Sheng Yong <shengyong@oppo.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>