clockintr_dispatch: copy cl_func to cq_shadow.cl_func before calling it

clockintr_dispatch: copy cl_func to cq_shadow.cl_func before calling it

When we add support for disestablishing clockintrs in the near future,
the current call to cl->cl_func will be in a race with free(9) on
another CPU.  Copying cl_func to cq_shadow.cl_func before leaving the
mutex and then calling the copy eliminates the race.

Correcting the race has a negligible performance impact and simplifies
the forthcoming clockintr_disestablish() patch.

Requested by dlg@ back in March.