vsock_diag_test: remove free_sock_stat() call in test_no_sockets

In `test_no_sockets` we don't expect any sockets, indeed
check_no_sockets() prints an error and exits if `sockets` list is
not empty, so free_sock_stat() call is unnecessary since it would
only be called when the `sockets` list is empty.

This was discovered by a strange warning printed by gcc v11.2.1:
  In file included from ../../include/linux/list.h:7,
                   from vsock_diag_test.c:18:
  vsock_diag_test.c: In function ‘test_no_sockets’:
  ../../include/linux/kernel.h:35:45: error: array subscript ‘struct vsock_stat[0]’ is partly outside array bound
  s of ‘struct list_head[1]’ [-Werror=array-bounds]
     35 |         const typeof(((type *)0)->member) * __mptr = (ptr);     \
        |                                             ^~~~~~
  ../../include/linux/list.h:352:9: note: in expansion of macro ‘container_of’
    352 |         container_of(ptr, type, member)
        |         ^~~~~~~~~~~~
  ../../include/linux/list.h:393:9: note: in expansion of macro ‘list_entry’
    393 |         list_entry((pos)->member.next, typeof(*(pos)), member)
        |         ^~~~~~~~~~
  ../../include/linux/list.h:522:21: note: in expansion of macro ‘list_next_entry’
    522 |                 n = list_next_entry(pos, member);                       \
        |                     ^~~~~~~~~~~~~~~
  vsock_diag_test.c:325:9: note: in expansion of macro ‘list_for_each_entry_safe’
    325 |         list_for_each_entry_safe(st, next, sockets, list) {
        |         ^~~~~~~~~~~~~~~~~~~~~~~~
  In file included from vsock_diag_test.c:18:
  vsock_diag_test.c:333:19: note: while referencing ‘sockets’
    333 |         LIST_HEAD(sockets);
        |                   ^~~~~~~
  ../../include/linux/list.h:23:26: note: in definition of macro ‘LIST_HEAD’
     23 |         struct list_head name = LIST_HEAD_INIT(name)

It seems related to some compiler optimization and assumption
about the empty `sockets` list, since this warning is printed
only with -02 or -O3. Also removing `exit(1)` from
check_no_sockets() makes the warning disappear since in that
case free_sock_stat() can be reached also when the list is
not empty.
Reported-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20211014152045.173872-1-sgarzare@redhat.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/vsock/vsock_diag_test.c

@@ -332,8 +332,6 @@ static void test_no_sockets(const struct test_opts *opts)
 	read_vsock_stat(&sockets);
 
 	check_no_sockets(&sockets);
-
-	free_sock_stat(&sockets);
 }
 
 static void test_listen_socket_server(const struct test_opts *opts)