f2fs: add missing f2fs_balance_fs in f2fs_zero_range

f2fs_balance_fs should be called in between node page updating, otherwise
node page count will exceeded far beyond watermark of triggering
foreground garbage collection, result in facing high risk of hitting LFS
allocation failure.

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