zebra: use SO_RCVBUFFORCE for netlink socket

so net.core.rmem_max must not be adjusted. Requires
linux kernel >= 2.6.14, falls back to SO_RCVBUF on error

Signed-off-by: Ulrich Weber <ulrich.weber@sophos.com>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/doc/zebra.8 b/doc/zebra.8
index 23703e7..a40909a 100644
--- a/doc/zebra.8
+++ b/doc/zebra.8
@@ -80,7 +80,7 @@
 messages in zebra log, you are in trouble.
 
 Solution is to increase receive buffer of netlink socket. Note that kernel
-doesn't allow to increase it over maximum value defined in
+< 2.6.14 doesn't allow to increase it over maximum value defined in
 \fI/proc/sys/net/core/rmem_max\fR. If you want to do it, you have to increase
 maximum before starting zebra.