Move the auto-sizing of the callout array from init_param2() to
kern_timeout_callwheel_alloc() where it is actually used.

This is a mechanical move and no tuning parameters are changed.

The pre-allocated callout array is only used for legacy timeout(9)
calls and is only allocated and active on cpu0.  Eventually all
remaining users of timeout(9) should switch to the callout_* API.

Reviewed by:	davide