Only try to allocate MSRs once instead of for every CPU.

This assumes the MSRs are the same on all CPUs which is currently
true. P4-HT is a special case for different SMT threads, but the code
always saves/restores all MSRs so it works identical.
Signed-off-by: NAndi Kleen <ak@suse.de>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>