libmount: add X-mount.subdir=

libmount: add X-mount.subdir=

Like btrfs subvol=, but more generic and for all filesystems. This
feature is marked as EXPERIMENTAL (may be removed).

 # mount /dev/sdc /mnt/test -o X-mount.subdir=AAA

 # findmnt /dev/sdc
 TARGET    SOURCE         FSTYPE OPTIONS
 /mnt/test /dev/sdc[/AAA] ext4   rw,relatime,stripe=512

Implemented as:

 * open current namespace from /proc/self/ns/mnt
 * mkdir /tmp/mount/mount.<pid>
 * unshare mount namespace
 * create mount node from /tmp/mount/mount.<pid> by mount(MS_BIND)
 * mark /tmp/mount/mount.<pid> private by mount(MS_PRIVATE|MS_REC)
 * bind mount /tmp/mount/mount.<pid>/subdir to <target>
 * umount /tmp/mount/mount.<pid>
 * rmdir /tmp/mount/mount.<pid>
 * setns() to the original namespace

Note that /tmp/mount/mount.<pid> conversion to mount node and call for
MS_PRIVATE are visible (propagated to the system) if /tmp is a shared
filesystem, the rest (all operations with the desired filesystem) is
atomic for a parental namespace.

Maybe one day it will be possible to reimplement it in more
elegant way with new mount kernel APIs (open_tree(), etc.).

Fixes: https://github.com/karelzak/util-linux/issues/1103
Signed-off-by: Karel Zak <kzak@redhat.com>