Fix race condition in catopen(3).

The current code uses a rwlock to protect the cached list, which
in turn holds a list of catentry objects, and increments reference
count while holding only read lock.

Fix this by converting the reference counter to use atomic operations.

While I'm there, also perform some clean ups around memory operations.

PR: 202636
Reported by: Henry Hu <henry.hu.sh@gmail.com>
Reviewed by: markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D24095