graphdriver/btrfs: use free wrapper consistently

While the Cgo in this entire file is quite questionable, that is a task
for another day.

Signed-off-by: Bjorn Neergaard <bneergaard@mirantis.com>
(cherry picked from commit d3778d65fa)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

daemon/graphdriver/btrfs/btrfs.go

@@ -233,7 +233,7 @@ func subvolSnapshot(src, dest, name string) error {
 
 	var cs = C.CString(name)
 	C.set_name_btrfs_ioctl_vol_args_v2(&args, cs)
-	C.free(unsafe.Pointer(cs))
+	free(cs)
 
 	_, _, errno := unix.Syscall(unix.SYS_IOCTL, getDirFd(destDir), C.BTRFS_IOC_SNAP_CREATE_V2,
 		uintptr(unsafe.Pointer(&args)))