f2fs: fix to do f2fs_balance_fs in f2fs_map_blocks correctly

If we preallocate blocks with f2fs_reserve_blocks in f2fs_map_blocks, we
should call f2fs_balance_fs for checking and reclaiming space, fix it.

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