x86/nmi: Improve NMI duration console printouts

Convert the last remaining printk() in nmi.c to pr_info(). Along with
it, use timespec macros to calculate the NMI handler duration.

Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Link: https://lore.kernel.org/r/20250327234629.3953536-10-sohil.mehta@intel.com