rstp: Fix daemon exit unexpected

When using rstpctl command to stop the rstp service, the daemon will
stop unexpectedly.
it exits because the rcvmsg returns error that no buffer space available, 
but this is not a critical issue, let it ignore this and continue.

Change-Id: Ia20978a9dd846c24a6fef9604346f1356f44adc3
Signed-off-by: Ken Zhu <guigenz@codeaurora.org>

rstp/patches/99-fix-exit-unexpected.patch

+diff -Nur a/brmon.c rstp-2011-10-11/brmon.c
+--- a/brmon.c	2019-05-17 11:33:57.554922414 -0700
++++ rstp-2011-10-11/brmon.c	2019-05-17 11:34:12.734922796 -0700
+@@ -164,7 +164,6 @@
+ {
+   if (rtnl_listen(&rth, dump_msg, stdout) < 0) {
+     fprintf(stderr, "Error on bridge monitoring socket\n");
+-    exit(-1);
+   }
+ }
+