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>