These warnings become somewhat more informative when they include the
MTU value that could not be set and not just the errno.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Link: https://lore.kernel.org/r/20201205133944.10182-1-rasmus.villemoes@prevas.dk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>