x86_64: Don't use softirq safe locks in smp_call_function

It is not fully softirq safe anyways.

Can't do a WARN_ON unfortunately because it could trigger in the
panic case.

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>