x86/xen: Return from panic notifier

Currently execution of panic() continues until Xen's panic notifier
(xen_panic_event()) is called at which point we make a hypercall that
never returns.

This means that any notifier that is supposed to be called later as
well as significant part of panic() code (such as pstore writes from
kmsg_dump()) is never executed.

There is no reason for xen_panic_event() to be this last point in
execution since panic()'s emergency_restart() will call into
xen_emergency_restart() from where we can perform our hypercall.

Nevertheless, we will provide xen_legacy_crash boot option that will
preserve original behavior during crash. This option could be used,
for example, if running kernel dumper (which happens after panic
notifiers) is undesirable.

Reported-by: James Dingwall <james@dingwall.me.uk>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Juergen Gross <jgross@suse.com>

Documentation/admin-guide/kernel-parameters.txt

@@ -5267,6 +5267,10 @@
 				the unplug protocol
 			never -- do not unplug even if version check succeeds
 
+	xen_legacy_crash	[X86,XEN]
+			Crash from Xen panic notifier, without executing late
+			panic() code such as dumping handler.
+
 	xen_nopvspin	[X86,XEN]
 			Disables the ticketlock slowpath using Xen PV
 			optimizations.