cpuset: Handle CPU_WHICH_TIDPID wherever cpuset_which() is called.

cpuset_which() resolves the argument pair which and id and returns references
to an appropriate resources. To avoid leaking resources or accessing unresolved
references to a resources handle new which CPU_WHICH_TIDPID wherever
cpuset_which() is called.
To avoid code duplication cpuset_which2() has been added.

Reported by: syzbot+331e8402e0f7347f0f2a@syzkaller.appspotmail.com
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D38272
MFC after: 2 weeks