IB/ipoib: When given an invalid UD MTU, give debug msg

In datagram mode, the IB UD (Unreliable Datagram) transport is used
so the MTU of the interface is equal to the IB L2 MTU minus the
IPoIB encapsulation header. Any request to change the MTU value
above the maximum range will change the MTU to the max allowed, but
will not show any warning message. An ipoib_warn is issued in such
cases, letting the user know that even though the value is legal,
it can't be currently applied.

Signed-off-by: Feras Daoud <ferasda@mellanox.com>
Signed-off-by: Noa Osherovich <noaos@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>