BUG/MINOR: backend: fix wrong BUG_ON for avail conn

BUG/MINOR: backend: fix wrong BUG_ON for avail conn

Idle connections are both stored in an idle/safe tree and in an idle
list. The list is used as a secondary storage to be able to retrieve
them by usage order.

If a connection is moved into the available tree, it must not be present
in the idle list. A BUG_ON() was written to check this but was placed at
the wrong code section. Fix this by removing the misplaced one and write
new ones for avail_conns tree insertion and lookup.

The impact of this bug is minor as the misplaced BUG_ON() did not seem
to be triggered.

No need to backport.