cpumask: convert arch/x86/kernel/nmi.c's backtrace_mask to a cpumask_var_t

Impact: cleanup, reduce memory usage for CONFIG_CPUMASK_OFFSTACK=y

I *think* every path calls check_nmi_watchdog before using the
watchdog, so that's the right place for the initialization.

If that's wrong, we'll get a nice NULL-deref with
CONFIG_CPUMASK_OFFSTACK=y, and have uncovered another bug.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>