kexec: allow kdump with crash_kexec_post_notifiers

kexec: allow kdump with crash_kexec_post_notifiers

If a crash kernel is loaded, do not crash the running domain.  This is
needed if the kernel is loaded with crash_kexec_post_notifiers, because
panic notifiers are run before __crash_kexec() in that case, and this
Xen hook prevents its being called later.

[akpm@linux-foundation.org: build fix: unconditionally include kexec.h]
Link: http://lkml.kernel.org/r/20160713122000.14969.99963.stgit@hananiah.suse.cz
Signed-off-by: Petr Tesarik <ptesarik@suse.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>