fix new volume populate

[bergwolf]

Allow to populate new volumes if the target volume:

1. does not exist
2. has only . and ..
3. has only ., .. and an empty lost+found dir

[bergwolf]

Before patch, empty nfs volume is not populated.

[~]$pkt run --name optiways-storage -d -v optiways:/fio hyperhq/nfs-server
447af39705df0f61a042078479d82d11dc7482835dd37353d5c5edfdba2b052c
[~]$pkt run --rm -it --volumes-from optiways-storage --name optiways-grpc-server bergwolf/ubuntu-fio
root@20bb4831f946:/# ls fio
root@20bb4831f946:/# df
Filesystem       1K-blocks   Used Available Use% Mounted on
/dev/sda          10190100 224320   9425108   3% /
devtmpfs            249952      0    249952   0% /dev
tmpfs               253156      0    253156   0% /dev/shm
rootfs              249952  16164    233788   7% /etc/hostname
172.16.0.35:/fio  10190848  36864   9613312   1% /fio
share_dir             1024      4      1020   1% /etc/hosts

After patch, empty nfs volume is populated with data on rootfs.

[~]$pkt run --name optiways-storage -d -v optiways:/fio hyperhq/nfs-server
0d9bf919b6a15d06237e3fad6fbb92217d1dcd9357d617785d6456826ba3342e
[~]$pkt run --rm -it --volumes-from optiways-storage --name optiways-grpc-server bergwolf/ubuntu-fio
root@59f495c1850f:/# ls fio
128k-read.fio  128k-write.fio  4k-read.fio  4k-write.fio  dev.fio  fs.fio  largefile.fio
root@59f495c1850f:/# df
Filesystem       1K-blocks   Used Available Use% Mounted on
/dev/sda          10190100 224320   9425108   3% /
devtmpfs            249952      0    249952   0% /dev
tmpfs               253156      0    253156   0% /dev/shm
rootfs              249952  16168    233784   7% /etc/hostname
172.16.0.29:/fio  10190848  36864   9613312   1% /fio
share_dir             1024      4      1020   1% /etc/hosts

[gnawux]

LGTM