batman-adv: Fix lock assert after fragmentation change

The automatic recalculation of the maximum allowed MTU is usually triggered
by code sections which are already rtnl lock protected by callers outside
of batman-adv. But when the fragmentation setting is changed via
batman-adv's own batadv genl family, then the rtnl lock is not yet taken.

But dev_set_mtu requires that the caller holds the rtnl lock because it
uses netdevice notifiers. And this code will then fail the check for this
lock:

  RTNL: assertion failed at net/core/dev.c (1953)

Fixes: e7ee4c55

 ("batman-adv: update to version 2023.2")
Signed-off-by: Sven Eckelmann <sven@narfation.org>