x86 kdump: make kdump_nmi_callback() a function ptr on crash_nmi_callback()

Impact: extend nmi_shootdown_cpus() with a callback

The reboot code will use a different function on crash_nmi_callback().
Adding a function pointer parameter to nmi_shootdown_cpus() for that.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>