KVM: x86: remove bogus user-triggerable WARN_ON

The WARN_ON is essentially comparing a user-provided value with 0.  It is
trivial to trigger it just by passing garbage to KVM_SET_CLOCK.  Guests
can break if you do so, but the same applies to every KVM_SET_* ioctl.
So, if it hurts when you do like this, just do not do it.

Reported-by:  <syzbot+00be5da1d75f1cc95f6b@syzkaller.appspotmail.com>
Fixes: 9446e6fc

 ("KVM: x86: fix WARN_ON check of an unsigned less than zero")
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>