vsock: always call vsock_init_tables()

Although CONFIG_VSOCKETS_DIAG depends on CONFIG_VSOCKETS,
vsock_init_tables() is not always called, it is called only
if other modules call its caller. Therefore if we only
enable CONFIG_VSOCKETS_DIAG, it would crash kernel on uninitialized
vsock_bind_table.

This patch fixes it by moving vsock_init_tables() to its own
module_init().

Fixes: 413a4317

 ("VSOCK: add sock_diag interface")
Reported-by: syzkaller bot
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Jorgen Hansen <jhansen@vmware.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>