pass upr_mutation and not #doc to xmem backend

Change-Id: I732ec6d6e30cccbecdd8398a5536260cb2dd6c3a
Reviewed-on: http://review.couchbase.org/37559
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>