x86: introduce a nmi_count tracking variable

x86: introduce a nmi_count tracking variable

This is modeled after the irq_count variable, and is used to account
for all the NMIs handled by the system.

This will allow to repurpose the nmi_count() helper so it can be used
in a similar manner as local_irq_count(): account for the NMIs
currently in service.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>