x86/crash: Indicate how well nmi_shootdown_cpus() managed to do

x86/crash: Indicate how well nmi_shootdown_cpus() managed to do

Having nmi_shootdown_cpus() report which pcpus failed to be shot down is a
useful debugging hint as to what possibly went wrong (especially when the
crash logs seem to indicate that an NMI timeout occurred while waiting for one
of the problematic pcpus to perform an action).

This is achieved by swapping an atomic_t count of unreported pcpus with a
cpumask.  In the case that the 1 second timeout occurs, use the cpumask to
identify the problematic pcpus.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>