According to the more information about this error link:
"It will also occur during backup if moodle database user has not sql privilege: create temporary tables."
Does the user set up to access the moodle DB have access rights enough to create temporary tables?
http://www.liquidweb.com/kb/grant-permissions-to-a-mysql-user-on-linux-via-command-line/
or use whatever tool you have to access the DB - phpmyadmin? - and check grants for the DB user.
If that is not changed, expect future issues even if you are successful in upgrading.
Looks like a large mdl_sessions table has issues and that your site is using the DB for session information. With whatever tool you have to interact with the DB for Moodle, remove all rows to mdl_sessions ... leave the table but empty it.
And, edit your config.php file to insert the following:
$CFG->dbsessions= '0';
That should set the moodle to use files based session recording ... not DB. A 'sessions' directory should be created by Moodle in moodledata the next time anyone attempts login.
Login to the 2.2 moodle if you can. Then check for a sessions file in moodledata/sessions to assure that is happening.
Then try the upgrade again.
Also, assume the site has met the version requirements for 2.7:
https://docs.moodle.org/dev/Moodle_2.7_release_notes#Server_requirements
'spirit of sharing', Ken
by Ken Task.