f2fs: data: fix warning Using plain integer as NULL pointer

changed passing function argument "0 to NULL" to fix below sparse
warning

fs/f2fs/data.c:426:47: warning: Using plain integer as NULL pointer
Signed-off-by: Hariprasad Kelam <hariprasad.kelam@gmail.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/data.c

@@ -428,7 +428,7 @@ static void __submit_merged_write_cond(struct f2fs_sb_info *sbi,
 
 void f2fs_submit_merged_write(struct f2fs_sb_info *sbi, enum page_type type)
 {
-	__submit_merged_write_cond(sbi, NULL, 0, 0, type, true);
+	__submit_merged_write_cond(sbi, NULL, NULL, 0, type, true);
 }
 
 void f2fs_submit_merged_write_cond(struct f2fs_sb_info *sbi,