f2fs: do not assign a new segment for dio under space shortage

If there is not enough free segment, we should not assign a new segment
explicitly. Otherwise, we can run out of free segment.
Reviewed-by: NChao Yu <chao2.yu@samsung.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -1225,7 +1225,8 @@ void allocate_data_block(struct f2fs_sb_info *sbi, struct page *page,
 	mutex_lock(&sit_i->sentry_lock);
 
 	/* direct_io'ed data is aligned to the segment for better performance */
-	if (direct_io && curseg->next_blkoff)
+	if (direct_io && curseg->next_blkoff &&
+				!has_not_enough_free_secs(sbi, 0))
 		__allocate_new_segments(sbi, type);
 
 	*new_blkaddr = NEXT_FREE_BLKADDR(sbi, curseg);