net: tcp: Handle case when RST is received in any state

We must check if we receive RST in any of the TCP states.
If we do not do this, then the net_context might leak as it
would never be released in some of the states. Receiving RST
in any TCP state is not described in TCP state diagram but is
described in RFC 793 which says in chapter "Reset Processing"
that system "...aborts the connection and advises the user and
goes to the CLOSED state."

We need to also validate the received RST and accept only those
TCP reset packets that contain valid sequence number.

The validate_state_transitions() function is also changed to
accept CLOSED state transition from various other states.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>