vsock: reject negative tv_usec in SO_VM_SOCKETS_CONNECT_TIMEOUT

The timeval validation checks tv_usec < USEC_PER_SEC but not
tv_usec >= 0, so a negative tv_usec makes the timeout computation
wrap into a bogus value, later tripping "schedule_timeout: wrong
timeout value" instead of being rejected.

Fixes: d021c344051a ("VSOCK: Introduce VM Sockets")
Signed-off-by: Nguyen Dinh Phi <phind.uet@gmail.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Bobby Eshleman <bobbyeshleman@meta.com>
Link: https://patch.msgid.link/20261030064428.73930-1-phind.uet@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>