libhtr: pthread_attr_setaffinity_np(): Replace calloc() with malloc()

Using calloc() instead of malloc() is useless here since the allocated
memory is to be wholly crushed by the memcpy() call that follows.

Suggested by: kib
Reviewed by: emaste, kib
Approved by: emaste (mentor)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D43328