Fix max_ncpus definition.

Fix max_ncpus definition.

It was being defined as the constant 64 and at first I changed it to be
NR_CPUS instead.

However, NR_CPUS can be a large value on recent kernels (4096), and this
may cause too large kmem allocations to happen.

Therefore, now we use num_possible_cpus(), which should return a (typically)
small value which represents the maximum number of CPUs than can be brought
online in the running hardware (this value is determined at boot time by
arch-specific kernel code).

Signed-off-by: Ricardo M. Correia <ricardo.correia@oracle.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>