umount: restrict non-root users to mountpoint paths only

umount: restrict non-root users to mountpoint paths only

Non-root users should only specify mountpoints when calling umount.
Device names, tags (LABEL=, UUID=, etc.) and loop device resolution
are disabled for unprivileged users to avoid processing untrusted
input through tag resolution code paths before permission checks.

In umount.c, reject tag arguments and disable swapmatch for
restricted users. In libmount, skip source and loopdev swapmatch
lookups for restricted contexts as defense in depth.

Signed-off-by: Karel Zak <kzak@redhat.com>