BUG/MEDIUM: list: fix incorrect pointer unlocking in LIST_DEL_LOCKED()

Injecting on a saturated listener started to exhibit some deadlocks
again between LIST_POP_LOCKED() and LIST_DEL_LOCKED(). Olivier found
it was due to a leftover from a previous debugging session. This patch
fixes it.

This will have to be backported if the other LIST_*_LOCKED() patches
are backported.

include/common/mini-clist.h

@@ -263,7 +263,7 @@ struct cond_wordlist {
 			        n2 = HA_ATOMIC_XCHG(&n->p, LLIST_BUSY);    \
 				if (n2 == LLIST_BUSY) {                    \
 					if (p2 != NULL)                    \
-						p2->n = (el);              \
+						p->n = p2;                 \
 					(el)->p = p;                       \
 					(el)->n = n;                       \
 					__ha_barrier_store();              \