fair_queue: Restrict push_priority_class
fair_queue: Restrict push_priority_class

The method adds priority class into dispatching queue. Nowadays it's
called internally after the class was dequeued, so the check whether
it's queued or not should be an assertion.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

syuu1228/seastar,scylladb/seastar,syuu1228/seastar,scylladb/seastar,syuu1228/seastar,avikivity/seastar,scylladb/seastar,avikivity/seastar,avikivity/seastar