KMountPoint: revert to parsing /dev/disk/by-{uuid,label}/ manually

Apparently libblkid is causing some performance issues when resolving
UUID/LABEL tags from fstab when the device that has that UUID/LABEL isn't
present.

Parsing /dev/disk/by-* is more basic, since it's a simple check resolving
some symlinks.

Thanks to the users in the bug report for the investigative work, because
personally I couldn't reproduce the problem on my machine no matter what I
tried.

BUG: 442106
FIXED-IN: 5.88