mpls: fix default mpls lb hash config

mpls: fix default mpls lb hash config

In case of multiple path within tunnel, mpls lookup node
computes lb hash with mpls_compute_flow_hash config value 0,
so only mpls label and l4 ports gets accounted, not 5-tuple.
This leads to flow traffic polarization and disbalance over
mpls paths.

Use mpls hash config from lb instead, usually it'll be
MPLS_FLOw_HASH_DEFAULT with 5-tuple plus flowlabel.
As optimization, fix flow hash reuse from the previous lookup
node if present, like ip_lookup does. Previously mpls lookup
always calcs the hash.
Test lb distribution for both cases.

Also, use the same flow hash hex format in ip4/ip6 and mpls
traces for easier reading, most code changes is due fixstyle
formatting.

Type: fix
Signed-off-by: Vladislav Grishenko <themiron@yandex-team.ru>
Change-Id: Ib89e1ab3edec14269866fe825a3e887d6c817b7c