staging/lustre/ptlrpc: Do not use deprecated cpus_* functions

As per Rusty Russel, cpus_* functions are deprecated.
When mixing cpumask_copy with cpus_weight, they operate on different
sized masks if CPUMASK_OFFSTACK is enabled, causing an
immediate assertion failure.
Copying of cpumasks by assignment is also not allowed now.

Additionally, in ptlrpc/service.c avoid the cpumask copies,
since we only use it to check how many siblings are there for
core #0 and nothing else.

Reported-by: Tyson Whitehead <twhitehead@gmail.com>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>