Filesystem: on stop, try umount directly, before scanning for users

48ed6e6d (Filesystem: improve stop-action and allow setting term/kill signals and signal_delay for large filesystems, 2023-07-04)
changed the logic from
"try umount; if that fails, find and kill users; repeat" to
"try to find and kill users; then try umount; repeat"

But even just walking /proc may take "a long time" on busy systems,
and may still turn up with "no users found".

It will take even longer for "force_umount=safe"
(observed 8 to 10 seconds just for "get_pids() with "safe" to return nothing)
than for "force_umount=yes" (still ~ 2 to 3 seconds),
but it will take "a long time" in any case.
(BTW, that may be longer than the hardcoded default of 6 seconds for "fast_stop",
which is also the default on many systems now)

If the dependencies are properly configured,
there should be no users left,
and the umount should just work.

Revert back to "try umount first", and only then try to find "rogue" users.