f2fs: avoid build warning

This patch removes build warning.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index 902c4c3..2c1e608 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -1715,7 +1715,7 @@
 	 * #2, flush sit entries to sit page.
 	 */
 	list_for_each_entry_safe(ses, tmp, head, set_list) {
-		struct page *page;
+		struct page *page = NULL;
 		struct f2fs_sit_block *raw_sit = NULL;
 		unsigned int start_segno = ses->start_segno;
 		unsigned int end = min(start_segno + SIT_ENTRY_PER_BLOCK,