f2fs: fix a problem of using memory after free

commit 7855eba4d6102f811b6dd142d6c749f53b591fa3 upstream.

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")
Signed-off-by: Yunlei He <heyunlei@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>