selftests/powerpc: Fix CPU affinity for child process

On systems with large number of cpus, test fails trying to set
affinity by calling sched_setaffinity() with smaller size for affinity
mask. This patch fixes it by making sure that the size of allocated
affinity mask is dependent on the number of CPUs as reported by
get_nprocs().

Fixes: 00b7ec5c

 ("selftests/powerpc: Import Anton's context_switch2 benchmark")
Reported-by: Shirisha Ganta <shiganta@in.ibm.com>
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Signed-off-by: Harish <harish@linux.ibm.com>
Reviewed-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Reviewed-by: Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200609081423.529664-1-harish@linux.ibm.com