net: bridge: add support for sticky fdb entries

[ Upstream commit 435f2e7cc0b783615d7fbcf08f5f00d289f9caeb ]

Add support for entries which are "sticky", i.e. will not change their port
if they show up from a different one. A new ndm flag is introduced for that
purpose - NTF_STICKY. We allow to set it only to non-local entries.

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>