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>

fs/f2fs/data.c

@@ -967,7 +967,7 @@ int f2fs_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
 	return ret;
 }
 
-struct bio *f2fs_grab_bio(struct inode *inode, block_t blkaddr,
+static struct bio *f2fs_grab_bio(struct inode *inode, block_t blkaddr,
 				 unsigned nr_pages)
 {
 	struct f2fs_sb_info *sbi = F2FS_I_SB(inode);