zswap: init under_reclaim

zswap: init under_reclaim

Bob Liu reported a memory leak in zswap.  This was due to the
under_reclaim field in the zbud header not being initialized
to 0, which resulted in zbud_free() not freeing the page
under the false assumption that the page was undergoing
zbud reclaim.

This patch properly initializes the under_reclaim field.

Signed-off-by: Seth Jennings <sjenning@linux.vnet.ibm.com>
Reported-by: Bob Liu <bob.liu@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>