A License check appears to already be running - Systemd sessions

Symptom

You will receive the following error when running a license check.

#  /usr/local/cpanel/cpkeyclt
Updating cPanel license...Done. Update Failed!
Error message:
    A License check appears to already be running.

Cause 

This happens when the inodes on /run are exhausted.

# df -i /run/
Filesystem Inodes  IUsed IFree IUse% Mounted on
tmpfs      458752 458752     0  100% /run

If this shows 100%, you can verify further with the command below, which will show an inode usage similar to the above.

# du --inodes --max-depth=0 /run/systemd/system/
407322 /run/systemd/system/

Resolution

You can reboot the server to clear the systemd session files.

How do I reboot my server?