udev: fix SECLABEL{selinux} issue (#15064)

Add SECLABEL{selinux}="some value" cause udevadm crash
systemd-udevd[x]: Worker [x] terminated by signal 11 (SEGV)

It happens since 25de7aa7 (Yu Watanabe 2019-04-25 01:21:11 +0200)
when udev rules processing changed to token model. Yu forgot store
attr to SECLABEL token so fix it.

(cherry picked from commit 0335d110)
(cherry picked from commit d58988be)
(cherry picked from commit 037a0fa5)