tcp: accept RST without ACK flag

tcp: accept RST without ACK flag

commit e566eb00dd7c2 (tcp: should drop incoming frames without ACK flag
set) added a regression on the handling of RST messages.

RST should be allowed to come even without ACK bit set. We validate
the RST by checking the exact sequence, as requested by RFC 793 and
5961 3.2, in tcp_validate_incoming()

Reported-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Tested-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: David S. Miller <davem@davemloft.net>