f2fs: fix non static symbol warning

Fixes the following sparse warning:

fs/f2fs/data.c:969:12: warning:
 symbol 'f2fs_grab_bio' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>