cpumask: implement cpumask_parse()

cpumask: implement cpumask_parse()

We have cpulist_parse() but not cpumask_parse().  Implement it using
bitmap_parse().

bitmap_parse() is weird in that it takes @len for a string in
kernel-memory which also is inconsistent with bitmap_parselist().
Make cpumask_parse() calculate the length and don't expose the
inconsistency to cpumask users.  Maybe we can fix up bitmap_parse()
later.

This will be used to expose workqueue cpumask knobs to userland via
sysfs.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>