f2fs: avoid build warning

This patch removes build warning.
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -1715,7 +1715,7 @@ void flush_sit_entries(struct f2fs_sb_info *sbi, struct cp_control *cpc)
 	 * #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,