multiqueue: Fix potential deadlock with parallel release_pad calls

multiqueue: Fix potential deadlock with parallel release_pad calls

Commit d3a66f9851ea introduced a potential deadlock with two parallel release_pad
calls, where one could release the main multiqueue lock (qlock) while still
holding the reconf_lock and then calling other routines which in some conditions
may try to acquire qlock again. The second release_pad could already acquire the
qlock and then start waiting on reconf_lock, which may never be possible because
because the first one isn't releasing it until it can acquire qlock.

Fix it by holding reconf_lock for the whole durationg of qlock, making this
particular deadlock impossible.

Fixes #1642

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3571>