f2fs: fix a problem of using memory after free

This patch fix a problem of using memory after free
in function __try_merge_extent_node.

Fixes: 0f825ee6 ("f2fs: add new interfaces for extent tree")
Cc: <stable@vger.kernel.org>
Signed-off-by: NYunlei He <heyunlei@huawei.com>
Reviewed-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/extent_cache.c

@@ -352,11 +352,12 @@ static struct extent_node *__try_merge_extent_node(struct inode *inode,
 	}
 
 	if (next_ex && __is_front_mergeable(ei, &next_ex->ei)) {
-		if (en)
-			__release_extent_node(sbi, et, prev_ex);
 		next_ex->ei.fofs = ei->fofs;
 		next_ex->ei.blk = ei->blk;
 		next_ex->ei.len += ei->len;
+		if (en)
+			__release_extent_node(sbi, et, prev_ex);
+
 		en = next_ex;
 	}