quota: adjust build-tags to allow build without CGO

This is to allow quota package (without tests) to be built without cgo.
makeBackingFsDev was used in helpers but not defined in projectquota_unsupported.go

Also adjust some GoDoc to follow the standard format.

Signed-off-by: Tibor Vass <tibor@docker.com>
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit 7cf079acdb88a5e66bf77e54b5dc30df859969bd)
Signed-off-by: Pete Woods <pete.woods@circleci.com>

quota/projectquota.go

@@ -396,9 +396,9 @@ func getDirFd(dir *C.DIR) uintptr {
 	return uintptr(C.dirfd(dir))
 }
 
-// Get the backing block device of the driver home directory
-// and create a block device node under the home directory
-// to be used by quotactl commands
+// makeBackingFsDev gets the backing block device of the driver home directory
+// and creates a block device node under the home directory to be used by
+// quotactl commands.
 func makeBackingFsDev(home string) (string, error) {
 	var stat unix.Stat_t
 	if err := unix.Stat(home, &stat); err != nil {

quota/testhelpers.go

@@ -1,4 +1,4 @@
-// +build linux
+// +build linux,!exclude_disk_quota,cgo
 
 package quota // import "github.com/docker/docker/quota"