Finish cpuset_getaffinity() after f35093f8

Split cpuset_getaffinity() into a two counterparts, where the
user_cpuset_getaffinity() is intended to operate on the cpuset_t from
user va, while kern_cpuset_getaffinity() expects the cpuset from kernel
va.
Accordingly, the code that clears the high bits is moved to the
user_cpuset_getaffinity(). Linux sched_getaffinity() syscall returns
the size of set copied to the user-space and then glibc wrapper clears
the high bits.

MFC after: 2 weeks

(cherry picked from commit d46174cd8838b86b9fe956b80c82bd238c302b2e)