cpumask: only allocate nr_cpumask_bits.

Now we'll find out the hard way if anyone has CPUMASK_OFFSTACK and is
returning these or assigning them.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

diff --cc include/linux/cpumask.h
Simple merge

diff --cc lib/cpumask.c
Simple merge