pkg/mount: mountinfo from specified pid

Signed-off-by: Vincent Batts <vbatts@redhat.com>
Merge pull request #8931 from vbatts/vbatts-mount_pid_mountinfo

pkg/mount: mountinfo from specified pid
vendor: update vendor'd libcontainer version

This patch updates the vendor'd libcontainer version, so that Docker can
take advantage of the updates to the `user` API.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)
*: transition to new libcontainer/user API

This patch fixes the compilation errors in Docker due to changes in the
libcontainer/user API. There is no functionality change due to this
patch.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)