net: tcp: Handle RST packet when multiple flags are set

After it sends SYN_ACK, there is a case that the client sends the packet
with both ACK and RST bits are set, and this packet needs to be handled
if the packet is valid.

   CLIENT                 SERVER
   ------                 ------
     |--------- SYN -------->|
     |<------ SYN_ACK -------|
     |------- ACK_RST ------>|
     |--------- SYN -------->|
     |<-------- ??? ---------|

This patch checks the RST bits even if other flags are set and process
the packet.

Signed-off-by: Tedd Ho-Jeong An <tedd.an@intel.com>