cpumask: use modern cpumask style in drivers/edac/amd64_edac.c

cpumask_t -> struct cpumask, and don't put one on the stack.  (Note: this
is actually on the stack unless CONFIG_CPUMASK_OFFSTACK=y).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>