libmount: accept X-mount.idmap=

libmount: accept X-mount.idmap=

This adds a new mount option X-mount.idmap. This mount option can be
used to create an idmapped mount.

An idmapped mount allows to change ownership of all files located under
a mount according to the ID-mapping associated with a user namespace.

The ownership change is tied to the lifetime and localized to the
relevant mount. The relevant ID-mapping can be specified in two ways:

* A user can specify the ID-mapping directly.
  The ID-mapping must be specified using the syntax
  id-type:id-mount:id-host:id-range
  Specifying "u" as the id-type prefix creates a UID-mapping, "g"
  creates a GID-mapping and omitting id-type or specifying "b"
  creates both a UID- and GID-mapping.
  The id-mount parameter indicates the starting ID in the new mount.
  The id-host parameter indicates the starting ID in the filesystem.
  The id-range parameter indicates how many IDs are to be mapped.
  It is possible to specify multiple ID-mappings.
  The individual ID-mappings must be separated by spaces.

  For example, the ID-mapping
  X-mount.idmap=u:1000:0:1 g:1001:1:2 5000:1000:2
  creates an idmapped mount where UID 0 is mapped to UID 1000, GID 1 is
  mapped to GUID 1001, GID 2 is mapped to GID 1002, UID and GID 1000 are
  mapped to 5000, and UID and GID 1001 are mapped to 5001 in the mount.

  When an ID-mapping is specified directly a new user namespace will be
  allocated with the requested ID-mapping.
  The newly created user namespace will be attached to the mount.

* A user can specify a user namespace file.
  The user namespace will then be attached to the mount and the
  ID-mapping of the user namespace will become the ID-mapping of the
  mount.
  For example, *X-mount.idmap=/proc/PID/ns/user* will attach the user
  namespace of the process PID to the mount.

Even more details about idmapped mounts can be found in the
mount_setattr(2) manpage of the linux-manpage project.

[kzak@redhat.com: - port Christian's patch to the new libmount hooks API]

Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Karel Zak <kzak@redhat.com>