fstrim: do not use Protect setting in systemd service

fstrim: do not use Protect setting in systemd service

The ProtectHome= and ProtectSystem= settings mounts all stuff for the service in read-only mode.

The fstrim ioctl operates on read-only mountpoint file descriptor, but
on some read-only filesystem the operation can fail, so since
2d22ac64e4 we check for read-only volumes and skip it.

References: Upstream: http://github.com/karelzak/util-linux/commit/2d22ac64e4d6e6732640f38b7232b5bcdc84a877
Addresses: https://github.com/karelzak/util-linux/issues/948
Signed-off-by: Karel Zak <kzak@redhat.com>