x86/nmi: Mark 'ignore_nmis' as __read_mostly

ignore_nmis is used in two distinct places:

 1. modified through {stop,restart}_nmi by alternative_instructions
 2. read by do_nmi to determine if default_do_nmi should be called or not

thus the access pattern conforms to __read_mostly and do_nmi() is a fastpath.

Signed-off-by: Kostenzer Felix <fkostenzer@live.at>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>