tcp: don't check mtu probe completion in the loop

It seems that no variables clash such that we couldn't do
the check just once later on. Therefore move it.

Also kill dead obvious comment, dead argument and add
unlikely since this mtu probe does not happen too often.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>