[NET]: Remove in-code externs for some functions from net/core/dev.c

Inconsistent prototype and real type for functions may have worse
consequences, than those for variables, so move them into a header.

Since they are used privately in net/core, make this file reside in
the same place.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>