sched/balancing: Switch the 'DEFINE_SPINLOCK(balancing)' spinlock into an 'atomic_t sched_balance_running' flag

The 'balancing' spinlock added in:

  08c183f3 ("[PATCH] sched: add option to serialize load balancing")

... is taken when the SD_SERIALIZE flag is set in a domain, but in reality it
is a glorified global atomic flag serializing the load-balancing of
those domains.

It doesn't have any explicit locking semantics per se: we just
spin_trylock() it.

Turn it into a ... global atomic flag. This makes it more
clear what is going on here, and reduces overhead and code
size a bit:

  # kernel/sched/fair.o: [x86-64 defconfig]

     text	   data	    bss	    dec	    hex	filename
    60730	   2721	    104	  63555	   f843	fair.o.before
    60718	   2721	    104	  63543	   f837	fair.o.after

Also document the flag a bit.

No change in functionality intended.
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Cc: Shrikanth Hegde <sshegde@linux.ibm.com>
Link: https://lore.kernel.org/r/20240308105901.1096078-2-mingo@kernel.org