sched: Make find_busiest_queue() a method

sched: Make find_busiest_queue() a method

Its a bit awkward but it was the least painful means of modifying the
queue selection. Used in a later patch to conditionally use a random
queue.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Turner <pjt@google.com>
Cc: Lee Schermerhorn <Lee.Schermerhorn@hp.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Hugh Dickins <hughd@google.com>
Link: http://lkml.kernel.org/n/tip-lfpez319yryvdhwqfqrh99f2@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>