Pass ploopDriver as a reference
Pass ploopDriver as a reference

Fixes the following legitimate 'go vet' warnings:

driver.go:79: newPloopDriver returns Lock by value: main.ploopDriver contains sync.RWMutex
driver.go:109: Create passes Lock by value: main.ploopDriver contains sync.RWMutex
driver.go:170: Remove passes Lock by value: main.ploopDriver contains sync.RWMutex
driver.go:206: Mount passes Lock by value: main.ploopDriver contains sync.RWMutex
driver.go:236: Unmount passes Lock by value: main.ploopDriver contains sync.RWMutex
driver.go:262: Get passes Lock by value: main.ploopDriver contains sync.RWMutex
driver.go:278: List passes Lock by value: main.ploopDriver contains sync.RWMutex
driver.go:309: Path passes Lock by value: main.ploopDriver contains sync.RWMutex
driver.go:326: volExist passes Lock by value: main.ploopDriver contains sync.RWMutex

Signed-off-by: Kir Kolyshkin <c0159612b0ef32139322233232df1a0d189c4b9b@openvz.org>

kolyshkin/docker-volume-ploop