util/lru: update c.head when deleting the most recently used entry

Fixes tailscale/corp#14747

Signed-off-by: David Anderson <danderson@tailscale.com>
Co-authored-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Signed-off-by: David Anderson <danderson@tailscale.com>

util/lru/lru.go

@@ -174,6 +174,9 @@ func (c *Cache[K, V]) deleteElement(ent *entry[K, V]) {
 	} else {
 		ent.next.prev = ent.prev
 		ent.prev.next = ent.next
+		if c.head == ent {
+			c.head = ent.next
+		}
 	}
 	delete(c.lookup, ent.key)
 }