net: bridge: fdb: convert added_by_external_learn to use bitops

[ Upstream commit b5cd9f7c42480ede119a390607a9dbe6263f6795 ]

Convert the added_by_external_learn field to a flag and use bitops.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: bee2ef946d31 ("net: bridge: br_fdb_external_learn_add(): always set EXT_LEARN")
Signed-off-by: Sasha Levin <sashal@kernel.org>