vsock: use sock_error() to consume sk_err after a failed connect

vsock_connect() returns sk_err to userspace but does not clear it:
  if (sk->sk_err) {
            err = -sk->sk_err;

For a blocking connect() the error has already been delivered as
connect()'s return value, so leaving it set causes subsequent operations
like poll()/epoll() to keep reporting POLLERR even though the connect
failure was already delivered.

The error should be consumed once it has been returned to userspace.
Switch to sock_error(), which reads and clears sk_err atomically,
matching the behavior of other protocol implementations such as
__inet_stream_connect().

Fixes: d021c344051a ("VSOCK: Introduce VM Sockets")
Tested-by: Wupeng Ma <mawupeng1@huawei.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Nguyen Dinh Phi <phind.uet@gmail.com>
Link: https://patch.msgid.link/20260813173024.2362935-4-phind.uet@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>