VSOCK: make af_vsock.ko removable again

VSOCK: make af_vsock.ko removable again

Commit 3fcb916cc5f27fe8ec363a4179afbee0eb3666e9 ("vsock: always call
vsock_init_tables()") introduced a module_init() function without a
corresponding module_exit() function.

Modules with an init function can only be removed if they also have an
exit function.  Therefore the vsock module was considered "permanent"
and could not be removed.

This patch adds an empty module_exit() function so that "rmmod vsock"
works.  No explicit cleanup is required because:

1. Transports call vsock_core_exit() upon exit and cannot be removed
   while sockets are still alive.
2. vsock_diag.ko does not perform any action that requires cleanup by
   vsock.ko.

Fixes: 3fcb916cc5f2 ("vsock: always call vsock_init_tables()")
Reported-by: Xiumei Mu <xmu@redhat.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Jorgen Hansen <jhansen@vmware.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Jorgen Hansen <jhansen@vmware.com>
Signed-off-by: David S. Miller <davem@davemloft.net>