mm: compaction: minimise the time IRQs are disabled while isolating pages for migration

compaction_alloc() isolates pages for migration in isolate_migratepages.
While it's scanning, IRQs are disabled on the mistaken assumption the
scanning should be short.  Tests show this to be true for the most part
but contention times on the LRU lock can be increased.  Before this patch,
the IRQ disabled times for a simple test looked like

  Total sampled time IRQs off (not real total time): 5493
  Event shrink_inactive_list..shrink_zone                  1596 us count 1
  Event shrink_inactive_list..shrink_zone                  1530 us count 1
  Event shrink_inactive_list..shrink_zone                   956 us count 1
  Event shrink_inactive_list..shrink_zone                   541 us count 1
  Event shrink_inactive_list..shrink_zone                   531 us count 1
  Event split_huge_page..add_to_swap                        232 us count 1
  Event save_args..call_softirq                              36 us count 1
  Eve...