net: bridge: explicitly zero is_sticky in fdb_create

We need to explicitly zero is_sticky when creating a new fdb, otherwise
we might get a stale value for a new entry.

Fixes: 435f2e7c ("net: bridge: add support for sticky fdb entries")
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>