crashkernel: Add ExecStop

Add ExecStop to unload the panic kernel.

"systemctl stop crashkernel" can be used to disable the panic reboot and
free the memory.

"systemctl restart crashkernel" can be used to reload the panic kernel.

misc_systemd_units/crashkernel.service

@@ -7,6 +7,7 @@ StandardOutput=kmsg
 StandardError=kmsg
 Type=oneshot
 ExecStart=/usr/sbin/kexec -p /boot/bzImage.crash --initrd=/boot/grub/initramfs.igz --command-line="root=LABEL=root ro console=ttyS0,115200n8 console=ttyS1,115200n8 console=tty0 irqpoll maxcpus=1 reset_devices CRASH"
+ExecStop=/usr/sbin/kexec -p -u
 RemainAfterExit=yes
 
 [Install]