Revert "f2fs: put allocate_segment after refresh_sit_entry"

This reverts commit 3436c4bdb30de421d46f58c9174669fbcfd40ce0.

This makes a leak to register dirty segments. I reproduced the issue by
modified postmark which injects a lot of file create/delete/update and
finally triggers huge number of SSR allocations.

Cc: <stable@vger.kernel.org> # v4.10+
[Jaegeuk Kim: Change missing incorrect comment]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -1881,15 +1881,14 @@ void allocate_data_block(struct f2fs_sb_info *sbi, struct page *page,
 
 	stat_inc_block_count(sbi, curseg);
 
+	if (!__has_curseg_space(sbi, type))
+		sit_i->s_ops->allocate_segment(sbi, type, false);
 	/*
-	 * SIT information should be updated before segment allocation,
-	 * since SSR needs latest valid block information.
+	 * SIT information should be updated after segment allocation,
+	 * since we need to keep dirty segments precisely under SSR.
 	 */
 	refresh_sit_entry(sbi, old_blkaddr, *new_blkaddr);
 
-	if (!__has_curseg_space(sbi, type))
-		sit_i->s_ops->allocate_segment(sbi, type, false);
-
 	mutex_unlock(&sit_i->sentry_lock);
 
 	if (page && IS_NODESEG(type))