migration: stop compression to allocate and free memory frequently

migration: stop compression to allocate and free memory frequently

Current code uses compress2() to compress memory which manages memory
internally, that causes huge memory is allocated and freed very
frequently

More worse, frequently returning memory to kernel will flush TLBs
and trigger invalidation callbacks on mmu-notification which
interacts with KVM MMU, that dramatically reduce the performance
of VM

So, we maintain the memory by ourselves and reuse it for each
compression

Reviewed-by: Peter Xu <[email protected]>
Reviewed-by: Jiang Biao <[email protected]>
Signed-off-by: Xiao Guangrong <[email protected]>
Message-Id: <20180330075128[email protected]>
Signed-off-by: Dr. David Alan Gilbert <[email protected]>