flow_dissect: call init_default_flow_dissectors() earlier

Andre Noll reported panics after my recent fix (commit 34fad54c
"net: __skb_flow_dissect() must cap its return value")

After some more headaches, Alexander root caused the problem to
init_default_flow_dissectors() being called too late, in case
a network driver like IGB is not a module and receives DHCP message
very early.

Fix is to call init_default_flow_dissectors() much earlier,
as it is a core infrastructure and does not depend on another
kernel service.

Fixes: 06635a35 ("flow_dissect: use programable dissector in skb_flow_dissect and friends")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Andre Noll <maan@tuebingen.mpg.de>
Diagnosed-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>