zebra: Change wording of duplicate kernel nhg flag

zebra: Change wording of duplicate kernel nhg flag

Change the wording of the flag indicating we have received
a nexthop group from the kernel with a different ID but
is fundamentally identical to one we already have.

It was colliding with a flag of similar name in the nexthop struct.

Change it from NEXTHOP_GROUP_DUPLICATE -> NEXTHOP_GROUP_UNHASHABLE
since it is in fact unhashable.

Also change the wording of functions and comments referencing the same
problem.

Signed-off-by: Stephen Worley <sworley@cumulusnetworks.com>