KVM: arm64: Inject UNDEF for a register trap without accessor

Configuring a register trap without specifying an accessor function is
abviously a bug. Instead of calling die() when that happens, let's be a
bit more helpful and print the register encoding. Also inject an
undefined instruction exception in the guest, similar to other unhandled
register accesses.

Signed-off-by: Alexandru Elisei <[e-mail]>
Reviewed-by: Marc Zyngier <[e-mail]>
Tested-by: Fuad Tabba <[e-mail]>
Reviewed-by: Fuad Tabba <[e-mail]>
Link: https://msgid.link/[e-mail]
Signed-off-by: Oliver Upton <[e-mail]>

arch/arm64/kvm/sys_regs.c

···
 	 * that we don't know how to handle. This certainly qualifies
 	 * as a gross bug that should be fixed right away.
 	 */
-	BUG_ON(!r->access);
+	if (!r->access) {
+		bad_trap(vcpu, params, r, "register access");
+		return;
+	}
 
 	/* Skip instruction if instructed so */
 	if (likely(r->access(vcpu, params, r)))