mm, compaction: reorder fields in struct compact_control

Patch series "try to reduce fragmenting fallbacks", v3.

Last year, Johannes Weiner has reported a regression in page mobility
grouping [1] and while the exact cause was not found, I've come up with
some ways to improve it by reducing the number of allocations falling
back to different migratetype and causing permanent fragmentation.

The series was tested with mmtests stress-highalloc modified to do
GFP_KERNEL order-4 allocations, on 4.9 with "mm, vmscan: fix zone
balance check in prepare_kswapd_sleep" (without that, kcompactd indeed
wasn't woken up) on UMA machine with 4GB memory.  There were 5 repeats
of each run, as the extfrag stats are quite volatile (note the stats
below are sums, not averages, as it was less perl hacking for me).

Success rate are the same, already high due to the low allocation order
used, so I'm not including them.

Compaction stats:
(the patches are stacked, and I haven't measured the non-functional-changes
patches separa...