batman-adv: enable MCAST by default at compile time

Thanks to rigorous testing in wireless community mesh networks several
issues with multicast entries in the translation table were found and
fixed in the last 1.5 years. Now we see the first larger networks
(a few hundred nodes) with a batman-adv version with multicast
optimizations enabled arising, with no TT / multicast optimization
related issues so far.

Therefore it seems safe to enable multicast optimizations by default.
Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/Kconfig

@@ -82,6 +82,7 @@ config BATMAN_ADV_NC
 config BATMAN_ADV_MCAST
 	bool "Multicast optimisation"
 	depends on BATMAN_ADV && INET && !(BRIDGE=m && BATMAN_ADV=y)
+	default y
 	help
 	  This option enables the multicast optimisation which aims to
 	  reduce the air overhead while improving the reliability of