vsock: fix possible infinite sleep in vsock_connectible_wait_data()

Currently vsock_connectible_has_data() may miss a wakeup operation
between vsock_connectible_has_data() == 0 and the prepare_to_wait().

Fix the race by adding the process to the wait queue before checking
vsock_connectible_has_data().

Fixes: b3f7fd54

 ("af_vsock: separate wait data loop")
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reported-by: Frédéric Dalleau <frederic.dalleau@docker.com>
Tested-by: Frédéric Dalleau <frederic.dalleau@docker.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>