linux/open_by_handle: support Docker containers without seccomp
linux/open_by_handle: support Docker containers without seccomp

On containers without seccomp, some overlay filesystems do not support
name_to_handle_at. With this patch:

    # ./open_by_handle.bin
    / does not support name_to_handle_at
    /dev: mount 289 type 1 size 12: 38 79 6e 2e 01 00 00 00 00 00 00 00
    ... open_by_handle_at denied. Need CAP_DAC_READ_SEARCH capability.
    /tmp does not support name_to_handle_at
    /etc/hostname: mount 398 type 1 size 8: 2a 39 0a 00 10 53 ac b4
    ... open_by_handle_at denied. Need CAP_DAC_READ_SEARCH capability.

    # mount
    overlay on / type overlay
    (rw,relatime,lowerdir=
    /var/lib/docker/overlay2/l/W5VMXDL62VCGUKHEFVRMYQEYHI:
    /var/lib/docker/overlay2/l/PVWN3QCRYFHX25IL55DXYZDWM4:
    /var/lib/docker/overlay2/l/GRYQATCVXINFTSJSCPCNZDVT27:
    /var/lib/docker/overlay2/l/BWND6SLW7P7P3CHBVQF24TOVUK:
    /var/lib/docker/overlay2/l/K2OCL3IRDAL5K6WV35IMWQRANM:
    /var/lib/docker/overlay2/l/2AG4IDOFMRUOHJP2FLPYW6MW2P,
    upperdir=/var/lib/docker/overlay2/4ed88b641755ef99a5f553e3424c9cae8477f077b149a7e370f65a5efc058aab/diff,
    workdir=/var/lib/docker/overlay2/4ed88b641755ef99a5f553e3424c9cae8477f077b149a7e370f65a5efc058aab/work,
    index=off)

fishilico/shared,fishilico/shared,fishilico/shared,fishilico/shared,fishilico/shared,fishilico/shared,fishilico/shared