f2fs: write missing last sum blk of file pinning section

While do not allocating a new section in advance for file pinning area, I
missed that we should write the sum block for the last segment of a file
pinning section.

Fixes: 9703d69d ("f2fs: support file pinning for zoned devices")
Signed-off-by: Daeho Jeong <daehojeong@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>