opt: [tcpip] disable IP reassembly and fragmentation on IOP

PS2 networking targets a local LAN with MTU=1500, and TCP
negotiates MSS=1460 so IP fragmentation never arises in the
common case. Turn off IP_REASSEMBLY and IP_FRAG to drop the
corresponding lwIP code paths and free their static pool slots
(MEMP_NUM_REASSDATA / MEMP_NUM_FRAG_PBUF).

Savings observed in ps2ip-nm.irx: 94805 -> 91509 bytes (-3.3 KB
of code), plus runtime pool memory that no longer needs to be
reserved. ps2link end-to-end test (execee over TCP) confirmed
unaffected.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: fjtrujy

iop/tcpip/tcpip-base/include/lwipopts.h

@@ -104,6 +104,25 @@
 	Up to TCP_SND_BUF * 2 segments may be transmitted at once, thanks to Nagle and Delayed Ack. */
 #define MEM_SIZE		(TCP_SND_BUF * 2)
 
+/*
+   -----------------------------------------------
+   ---------- IP options -------------------------
+   -----------------------------------------------
+*/
+/**
+ * IP_REASSEMBLY==1: Reassemble incoming fragmented IP packets.
+ * Disabled: PS2 networking targets a local LAN with MTU=1500, and
+ * TCP negotiates MSS=1460 so fragments never arise in the common
+ * case. Frees MEMP_NUM_REASSDATA / MEMP_NUM_FRAG_PBUF pool entries.
+ */
+#define IP_REASSEMBLY		0
+
+/**
+ * IP_FRAG==1: Fragment outgoing IP packets if their size exceeds MTU.
+ * Disabled: TCP_MSS=1460 ensures we never exceed Ethernet MTU=1500.
+ */
+#define IP_FRAG			0
+
 /*
    ------------------------------------------------
    ---------- Internal Memory Pool Sizes ----------