sctp: discard stale INIT after handshake completion

sctp: discard stale INIT after handshake completion

After an association reaches ESTABLISHED, the peer’s init_tag is already
known from the handshake. Any subsequent INIT with the same init_tag is
not a valid restart, but a delayed or duplicate INIT.

Drop such INIT chunks in sctp_sf_do_unexpected_init() instead of
processing them as new association attempts.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Link: https://patch.msgid.link/5788c76c1ee122a3ed00189e88dcf9df1fba226c.1777214801.git.lucien.xin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>