krpc: Ref cnt the client structures for TLS upcalls

A crash occurred during testing, where the client structures had
already been free'd when the upcall thread tried to lock them.

This patch acquires a reference count on both of the structures
and these are released when the upcall is done, so that the
structures cannot be free'd prematurely. This happened because
the testing is done over a very slow vpn.

Found during a IETF bakeathon testing event this week.

MFC after: 5 days