Avoid unnecessary automounting in KDiskFreeSpaceInfo::freeSpaceInfo

Previously, all unmounted autofs mountpoints were always mounted
by krunner/plasma-desktop on startup, defeating the purpose of
automounting. Let's ignore the unmounted filesystems instead when
gathering free space stats, just like the "df" utility does.

Everything still gets mounted properly on first real access.

v2: Use directory size instead of hardcoded "autofs" type

REVIEW:117044