sync_mountpoint: open path so that sync works

sync_mountpoint: open path so that sync works

sync_mountpoint takes a path, gets an open filedescriptor and calls
syncfs(2) on it.
by opening with O_PATH the syncfs call fails with EBADF (see open(2)).

found by running:
```
pkill -f 'pvedaemon worker';
strace -yyttT -s 512 -o /tmp/trace -fp $(pgrep -f pvedaemon$)
```

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>