x86: fix abuse of per_cpu_offset

Impact: bug fix

Don't use per_cpu_offset() to determine if it valid to access a
per-cpu variable for a given cpu number.  It is not a valid assumption
on x86-64 anymore. Use cpu_possible() instead.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>