net: bridge: fdb: br_fdb_update can take flags directly

If we modify br_fdb_update() to take flags directly we can get rid of
one test and one atomic bitop in the learning path.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>