net: Enable SYS_LIGHTWEIGHT_PROT

This option actually enables the use of the lwip_sys_mutex for
protecting concurrent access to such important lwIP resources as:
  select_cb_list (this is the one which orig flagged problem)
  sockets array
  mem stats (if enabled)
  heap (if LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT was non-zero)
  memp pool allocs/frees
  netif->loop_last pbuf linked list
  pbuf reference counts
  ...

I first noticed this issue when I hit a crash while slamming the net
stack with a large number of TCP packets (I was actually sending 1k
data buffers from the TCPEchoServer mbed sample.)  It crashed in the
last line of this code snippet from event_callback:
  for (scb = select_cb_list; scb != NULL; scb = scb->next) {
    if (scb->sem_signalled == 0) {

It was crashing because scb had an invalid address so it generated a
bus fault.  I figured that memory was either corrupted or there was
some kind of concurrency issue.  In trying to determine which, I wanted
to walk through the select_cb_list linked list and see where it was
corrupted:
    (gdb) p scb
    $1 = (struct lwip_select_cb *) 0x85100080
    (gdb) p select_cb_list
    $2 = (struct lwip_select_cb *) 0x0

That was interesting, the head of the linked list was now NULL but it
must have had a non-NULL value when this loop started running or we
would have never gotten to the point where we hit this crash.

This was starting to look like a concurrency issue since the linked
list was modified out from underneath this thread.  Looking through the
source code for this function, I saw use of macros like
SYS_ARCH_PROTECT and SYS_ARCH_UNPROTECT which looked like they should
be providing the thead synchronization.  I disassembled the
event_callback() function in the debugger and saw no signs of the
usage of synchronizition APIs that I expected.  A search
through the code for the definition of these SYS_ARCH_UN/PROTECT
macros led me to discovering that they were actually ignored unless an
implementation defined them itself (the mbed version doesn't do so) or
the SYS_LIGHTWEIGHT_PROT macro is set to non-zero (the mbed version
didn't do this either).  Flipping the SYS_LIGHTWEIGHT_PROT macro on in
lwipopts.h fixed the crash I kept hitting, increased the size of the
code a bit, and unfortunately slows things down a bit since it now
actually serializes access to these data structures by making calls
to the RTOS sync APIs.

Author: adamgreen

libraries/net/lwip/lwip/lwipopts.h

@@ -27,6 +27,8 @@
 #if NO_SYS == 0
 #include "cmsis_os.h"
 
+#define SYS_LIGHTWEIGHT_PROT        1
+
 #define LWIP_RAW                    0
 
 #define TCPIP_MBOX_SIZE             8