x86/mce/amd: Straighten CPU hotplug path

mce_threshold_create_device() hotplug callback runs on the plugged in
CPU so:

 - use this_cpu_read() which is faster
 - pass in struct threshold_bank **bp to threshold_create_bank() and
   instead of doing per-CPU accesses
 - Use rdmsr_safe() instead of rdmsr_safe_on_cpu() which avoids an IPI.

No functional changes.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200403161943.1458-6-bp@alien8.de