staging: zram: replace xvmalloc with zsmalloc

Replaces xvmalloc with zsmalloc as the compressed page allocator
for zram

Signed-off-by: Nitin Gupta <ngupta@vflare.org>
Acked-by: Seth Jennings <sjenning@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>