memstore: fix touch double-allocation

Reported-by: Allen Samuels <allen.samuels@sandisk.com>
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1211 from ceph/wip-memstore

memstore: fix touch double-allocation

Reviewed-by: Samuel Just <sam.just@inktank.com>