f2fs: fix to avoid accessing invalid fio in f2fs_allocate_data_block()

Callers may pass fio parameter with NULL value to f2fs_allocate_data_block(),
so we should make sure accessing fio's field after fio's validation check.

Fixes: f608c38c ("f2fs: clean up parameter of f2fs_allocate_data_block()")
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>