odhcp6c_find_entry: exclude priority from the list of fields that must match

odhcp6c_find_entry: exclude priority from the list of fields that must match

Priority of an entry can vary from one RA to another, but the entry
identity should be the same regardless of the priority declared in the
RA message handled in ra_process() at a time.

CDRouter for instance tests compliance of the device  to requirement
G-5 of RFC 7084 by provisioning initially the CPE with a valid default
route that has a medium preference after which it sends a low preference
RA with lifetime 0 to check that CPE will start sending RAs with
lifetime 0 on the LAN side. Because odhcp6c didn't matched the low
precedence default router entry with the odhcp6c_entry that was
previously stored with medium precedence, it will fail to remove the
existing STATE_RA_ROUTE default route, hence preventing odhcpd from
advertising RAs with lifetime 0 to LAN.

Signed-off-by: Alin Nastac <alin.nastac@gmail.com>