net: bridge: fdb: avoid two atomic bitops in br_fdb_external_learn_add()

If we setup the fdb flags prior to calling fdb_create() we can avoid
two atomic bitops when learning a new entry.

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