mm/balloon_compaction.c: don't zero ballooned pages

Revert commit bb01b64c ("mm/balloon_compaction.c: enqueue zero page
to balloon device")'

Zeroing ballon pages is rather time consuming, especially when a lot of
pages are in flight. E.g. 7GB worth of ballooned memory takes 2.8s with
__GFP_ZERO while it takes ~491ms without it.

The original commit argued that zeroing will help ksmd to merge these
pages on the host but this argument is assuming that the host actually
marks balloon pages for ksm which is not universally true.  So we pay
performance penalty for something that even might not be used in the end
which is wrong.  The host can zero out pages on its own when there is a
need.

[mhocko@kernel.org: new changelog text]
Link: http://lkml.kernel.org/r/1501761557-9758-1-git-send-email-wei.w.wang@intel.com
Fixes: bb01b64c

 ("mm/balloon_compaction.c: enqueue zero page to balloon device")
Signed-off-by: Wei Wang <wei.w.wang@intel.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: zhenwei.pi <zhenwei.pi@youruncloud.com>
Cc: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>