zebra: Fix crash when OOM happens.

zebra: Fix crash when OOM happens.

The hash key function choosen for mac vni's would tend
to clump the key value to the same number.  Use a better
hash key generator to spread the hash values out.

A bad hash key might lead to O(2^n) memory consumption
because the hash size is doubled, each time a backet
exceeds a predefined threshold.  This quickly leads
to OOM.  Fixing this issue by fixing the hash
key generation to actually spread the keys out.

Ticket: CM-17412
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>