don't merge when the asking region entirely exists in the caching device

[akiradeveloper]

If the read is partially hit, we always do merging: reading the backing data and read the cached data and then merge them.

        if (unlikely(dirtiness.data_bits != 255)) {

but if the wanting data is all found in the caching device, we don't need to merge like non-partial read-hit case.

[akiradeveloper]

I made a reproducer.

class REPRO_118 extends DMTestSuite {
  // TODO enable this once dm-flakey is fixed
  ignore("don't merge when the asking region entirely exists in the caching device") {
    slowDevice(Sector.M(128)) { _slow => Linear.Table(_slow).create { slow =>
      fastDevice(Sector.M(32)) { _fast => Linear.Table(_fast).create { fast =>
        Writeboost.sweepCaches(fast)
        Writeboost.Table(slow, fast).create { s =>
          val data = DataBuffer.random(Sector(4).toB.toInt)
          s.bdev.write(Sector(1), data)
          s.dropTransient()
          // the data is cached

          // we don't merge the backing data and the cached data
          // but instead directly read the cached data
          slow.reload(Flakey.Table(_slow, 0, 1))
          assert(s.bdev.read(Sector(1), Sector(4)) isSameAs data)
        }
      }}
    }}
  }
}

[akiradeveloper]

remove -f (with force flag) should remove the device forcibly without waiting the flushing completed. I should write a test.