f2fs: silence VM_WARN_ON_ONCE in mempool_alloc

Note that __GFP_ZERO is not supported for mempool_alloc,
which also documented in the mempool_alloc comments.
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/data.c

@@ -299,9 +299,10 @@ static inline void __submit_bio(struct f2fs_sb_info *sbi,
 		for (; start < F2FS_IO_SIZE(sbi); start++) {
 			struct page *page =
 				mempool_alloc(sbi->write_io_dummy,
-					GFP_NOIO | __GFP_ZERO | __GFP_NOFAIL);
+					      GFP_NOIO | __GFP_NOFAIL);
 			f2fs_bug_on(sbi, !page);
 
+			zero_user_segment(page, 0, PAGE_SIZE);
 			SetPagePrivate(page);
 			set_page_private(page, (unsigned long)DUMMY_WRITTEN_PAGE);
 			lock_page(page);