[CRIU] [PATCH v8 00/15] AutoFS mounts migration support

[Stanislav Kinsburskiy]

This patch set adds suport for AutoFS mount migration, including:
1) Direct mounts
2) Indirect mounts
3) Catatonic mounts

Note: systemd AutoFS logic breaks with this migration, because device value of
new mount differs to the one, which systemd collected, when it was created the
mount. Because of it systemd ignors any requests from kernel.

v8:
1) find_unused_fd() was optimized: if hint_fd is -1, last used fd + 1 is
returned without loop iteration.
2) Patch "mount: copy private field to "bind mount" mount_info structure" was
dropped as obsolete.
3) Some cleanup in functions naming.
4) Feature "autofs" was added to test descriptor.

v7:
1) Rebased on criu-dev branch
2) test attached

v6: indirect mounts dentries are now created within autofs code on autofs
mount.

v5: this version has significant changes in comparison to previous one:
1) Autofs data is migrated via AutofsEntry structure, which is nested to
MntEntry structure.
2) Dump stage is now only converts real pgrp into virtual one. All the collect
and control pipe check is done in "parse" stage.
3) All the autofs pipes are collected into a list. This list is checked on
packetized pipe dump to distinguish between autofs and other pipes (which are
still not supported).
4) Control pipe restore is now done by creating another pipe fd with custom
post_open callback instead of artificial files.
5) New list of objects and actors was introduced to create shared autofs data
after both mounts and shrared objects are restored.
6) Timeout restore and making mount catatonic is done right after mount pint
is created. It allows to avoid of autofs data objects creation and post_open
callback calling in case of catatonic mounts.

v4: mount info is now transfered in deducated autofs images.

v3: fixed write pipe end restore, when it was closed.