compressed in-memory swapping take2

Hi,

It allows creating a RAM based block device which acts as swap disk.
Pages swapped to this device are compressed and stored in memory itself.
This is a big win over swapping to slow hard-disk which are typically used
as swap disk. For flash, these suffer from wear-leveling issues when used
as swap disk - so again its helpful. For swapless systems, it allows more
apps to run.

* Changelog: take2 vs initial revision:
xvmalloc changes:
  - Use Linux kernel coding sytle for xvmalloc
  - Collapse all individual flag test/set/get to generic {test_set_get}_flag
  - Added BLOCK_NEXT() macro to reach next contiguous block
  - Other minor cleanups - no functional changes
compcache block device code:
  - compcache core changes due to change in xvmalloc interface names

Links to performance numbers, use cases can be found in original thread:
http://lkml.org/lkml/2009/3/17/116

Thanks to Christoph and Pekka for feedback on initial revision.

Thanks,
Nitin
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