x86/mcheck: allow varying bank counts per CPU

x86/mcheck: allow varying bank counts per CPU

Up to now we've been assuming that all CPUs would have the same number
of reporting banks. However, on upcoming AMD CPUs this isn't the case,
and one can observe

(XEN) mce.c:666: Different bank number on cpu <N>

indicating that Machine Check support would not be enabled on the
affected CPUs. Convert the count variable to a per-CPU one, and adjust
code where needed to cope with the values not being the same. In
particular the mcabanks_alloc() invocations during AP bringup need to
now allocate maximum-size bitmaps, because the truly needed size can't
be known until we actually execute on that CPU, yet mcheck_init() gets
called too early to do any allocations itself.

Take the liberty and also
- make mca_cap_init() static,
- replace several __get_cpu_var() uses when a local variable suitable
  for use with per_cpu() appears,
- correct which CPU's cpu_data[] entry x86_mc_msrinject_verify() uses,
- replace a BUG() by panic().

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>