fstrim: Add hardening settings to fstrim.service

fstrim: Add hardening settings to fstrim.service

This limits what the fstrim process has access to when it runs.

PrivateUsers can't be enabled because of:
"If this mode is enabled, all unit processes are run without privileges
in the host user namespace[...]"

Further improving this with additional option or making
things even tighter is most likely possible.

Signed-off-by: Andreas Henriksson <andreas@fatal.se>
Signed-off-by: Karel Zak <kzak@redhat.com>