[PATCH v5] z3fold: add shrinker

[Vitaly Wool]

This patch set implements shrinker for z3fold, preceded by some
code optimizations and preparations that I thought would be
reasonable to have as separate patches.

This patch set has been verified on x86 and on Qemu emulating
Versatile Express. fio results for the resulting code are:
Run status group 0 (all jobs):
  WRITE: io=3200.0MB, aggrb=13095KB/s, minb=3273KB/s, maxb=3284KB/s, mint=249447msec, maxt=250214msec

Run status group 1 (all jobs):
   READ: io=3200.0MB, aggrb=28992KB/s, minb=7248KB/s, maxb=7273KB/s, mint=112623msec, maxt=113021msec

Run status group 2 (all jobs):
   READ: io=1595.2MB, aggrb=8825KB/s, minb=2194KB/s, maxb=2224KB/s, mint=184517msec, maxt=185077msec
  WRITE: io=1604.9MB, aggrb=8879KB/s, minb=2207KB/s, maxb=2245KB/s, mint=184519msec, maxt=185079msec

Run status group 3 (all jobs):
   READ: io=1600.6MB, aggrb=8413KB/s, minb=2084KB/s, maxb=2132KB/s, mint=193286msec, maxt=194803msec
  WRITE: io=1599.5MB, aggrb=8406KB/s, minb=2099KB/s, maxb=2120KB/s, mint=193290msec, maxt=194825msec

Disk stats (read/write):
  zram0: ios=1636792/1638952, merge=0/0, ticks=169250/462410, in_queue=633700, util=85.33%

Just for comparison, zsmalloc gives slightly worse results:
Run status group 0 (all jobs):
  WRITE: io=3200.0MB, aggrb=12827KB/s, minb=3206KB/s, maxb=3230KB/s, mint=253603msec, maxt=255450msec

Run status group 1 (all jobs):
   READ: io=3200.0MB, aggrb=26184KB/s, minb=6546KB/s, maxb=6556KB/s, mint=124940msec, maxt=125144msec

Run status group 2 (all jobs):
   READ: io=1595.2MB, aggrb=8549KB/s, minb=2123KB/s, maxb=2162KB/s, mint=190151msec, maxt=191049msec
  WRITE: io=1604.9MB, aggrb=8601KB/s, minb=2145KB/s, maxb=2172KB/s, mint=190153msec, maxt=191051msec

Run status group 3 (all jobs):
   READ: io=1600.6MB, aggrb=8147KB/s, minb=2026KB/s, maxb=2049KB/s, mint=200339msec, maxt=201154msec
  WRITE: io=1599.5MB, aggrb=8142KB/s, minb=2023KB/s, maxb=2062KB/s, mint=200343msec, maxt=201158msec

Disk stats (read/write):
  zram0: ios=1637032/1639304, merge=0/0, ticks=175840/458740, in_queue=637140, util=82.48%

Signed-off-by: Vitaly Wool <vitalywool@xxxxxxxxx>