sched/balancing: Rename find_busiest_group() => sched_balance_find_src_group()

Make two naming changes:

1)
   Standardize scheduler load-balancing function names on the
   sched_balance_() prefix.

2)

   Similar to find_busiest_queue(), the find_busiest_group() naming
   has become a bit of a misnomer: the 'busiest' qualifier to this
   function was historically correct but in the current code
   in quite a few cases we will not pick the 'busiest' group - but the best
   (possible) group we can balance from based on a complex set of
   constraints.

So name it a bit more neutrally, similar to the 'src/dst' nomenclature
we are already using when moving tasks between runqueues, and also
use the sched_balance_ prefix: sched_balance_find_src_group().

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Link: https://lore.kernel.org/r/20240308111819.1101550-9-mingo@kernel.org