f2fs: zone: fix to wait completion of last bio in zone correctly

It needs to check last zone_pending_bio and wait IO completion before
traverse next fio in io->io_list, otherwise, bio in next zone may be
submitted before all IO completion in current zone.

Fixes: e067dc3c ("f2fs: maintain six open zones for zoned devices")
Cc: Daeho Jeong <daehojeong@google.com>
Signed-off-by: Chao Yu <chao@kernel.org>
Reviewed-by: Daeho Jeong <daehojeong@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>