BUG/MEDIUM: ring: write-lock the ring while attaching/detaching

BUG/MEDIUM: ring: write-lock the ring while attaching/detaching

The LIST_ADDQ() and LIST_DEL_INIT() calls made to attach/detach a waiter
to the ring were made under a read lock which was sufficient in front of
the writer's write lock. But it's not sufficient against other readers!
Thus theorically multiple "show events" on the same ring buffer on the
CLI could result in a crash, even though for now I couldn't manage to
reproduce it.

This fixes commit 1d181e489c ("MEDIUM: ring: implement a wait mode for
watchers") so it must be backported to 2.1, possibly further if the ring
code gets backported.