Fix the long term ULE load balancer so that it actually works. The
initial call to sched_balance() during startup is meant to initialize
balance_ticks, but does not actually do that since smp_started is
still zero at that time. Since balance_ticks does not get set,
there are no further calls to sched_balance(). Fix this by setting
balance_ticks in sched_initticks() since we know the value of
balance_interval at that time, and eliminate the useless startup
call to sched_balance(). We don't need to randomize the intial
value of balance_ticks.

Since there is now only one call to sched_balance(), we can hoist
the tests at the top of this function out to the caller and avoid
the overhead of the function call when running a SMP kernel on UP
hardware.

PR: 223914
Reviewed by: kib
MFC after: 2 weeks