Add cxgb_best_mtu() in libcxgb_cm.h to remove
it's duplicate definitions from cxgb4/cm.c and
cxgbit/cxgbit_cm.c
Signed-off-by: NVarun Prakash <varun@chelsio.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>