sched/balancing: Rename find_busiest_queue() => sched_balance_find_src_rq()

The find_busiest_queue() naming has two small quirks:

 - Scheduler functions that deal with runqueues usually have a rq_ prefix
   or _rq postfix, but this function has neither.

 - Plus the 'busiest' qualifier to this function was historically
   correct, but has become somewhat of a misnomer: in quite a few
   cases we will not pick the busiest runqueue - but the best
   (possible) runqueue we can balance tasks from. So name it a
   bit more neutrally, similar to the 'src/dst' nomenclature
   we are already using when moving tasks between runqueues.

To fix both quirks, and to standardize scheduler load-balancing
function names on the sched_balance_() prefix, rename the
function to sched_balance_find_src_rq().

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Link: https://lore.kernel.org/r/20240308111819.1101550-7-mingo@kernel.org