vsock/test: fix send_buf()/recv_buf() EINTR handling

vsock/test: fix send_buf()/recv_buf() EINTR handling

When send() or recv() returns -1 with errno == EINTR, the code skips
the break but still adds the return value to nwritten/nread, making it
decrease by 1. This leads to wrong buffer offsets and wrong bytes count.

Fix it by explicitly continuing the loop on EINTR, so the return value
is only added when it is positive.

Fixes: a8ed71a27ef5 ("vsock/test: add recv_buf() utility function")
Fixes: 12329bd51fdc ("vsock/test: add send_buf() utility function")
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Luigi Leonardi <leonardi@redhat.com>
Link: https://patch.msgid.link/20260403093251.30662-1-sgarzare@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>