core.packet: Extend max packets from 100K to 1M

Packets are allocated dynamically on demand and this is only an
artifical limit of "more than anybody should ever need" for catching
unbounded allocations (leaks). (The limit also serves to make the
freelist simple i.e. a fixed-size array.)

The previous value of 100,000 is too low to serve its purpose: it is
reasonable to allocate more than 100,000 packet buffers when managing
many network receive queues and wanting to avoid packet drops.

See discussion at:
#882 (comment)

src/core/packet.lua

@@ -22,7 +22,7 @@ local header_size = 8
 local max_payload = tonumber(C.PACKET_PAYLOAD_SIZE)
 
 -- Freelist containing empty packets ready for use.
-local max_packets = 1e5
+local max_packets = 1e6
 local packet_allocation_step = 1000
 local packets_allocated = 0
 local packets_fl = freelist.new("struct packet *", max_packets)
@@ -108,7 +108,8 @@ function length (p) return p.length end
 
 function preallocate_step()
    if _G.developer_debug then
-      assert(packets_allocated + packet_allocation_step <= max_packets)
+      assert(packets_allocated + packet_allocation_step <= max_packets,
+             "packet allocation overflow")
    end
 
    for i=1, packet_allocation_step do