The pmap update_page/invalidate_page/invalidate_all operations has to be
done only on active cpus. In the simplest case, if the process is not
active on any other CPUs, we can just do the operation on the current CPU.
This change replaces the call to smp_rendezvous() for these operations with
smp_rendezvous_cpus() in case there more than one active CPU, or with a direct
function call if there is just one active CPU.
This change give significant performance increase in fork/exec benchmarks
on XLR/XLS/XLP with 32 cpus.
Reviewed by: alc
cca2c4b446