batman-adv: Don't increase MTU when set by user

stable inclusion
from stable-v5.10.193
commit 7c09590883665bb6645deb3e33cab2c2eb2e1aa7
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I9399M

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7c09590883665bb6645deb3e33cab2c2eb2e1aa7



--------------------------------

commit d8e42a2b upstream.

If the user set an MTU value, it usually means that there are special
requirements for the MTU. But if an interface gots activated, the MTU was
always recalculated and then the user set value was overwritten.

The only reason why this user set value has to be overwritten, is when the
MTU has to be decreased because batman-adv is not able to transfer packets
with the user specified size.

Fixes: c6c8fea2 ("net: Add batman-adv meshing protocol")
Cc: stable@vger.kernel.org
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>