MB-11305 Fix mock_stats type

The type used by mock stats (mock_new_independent_stats /
mock_release_independent_stats) was incorrect - it should be
'struct mock_stats'

Change-Id: Ice731564fdd9dfbc19e4c6095947251a6fa11259
Reviewed-on: http://review.couchbase.org/37835
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>