net: tcp: Use our MTU to determine the mss for transmission

In the existing the value received from the other side by the TCP options
is used as MSS for transmission. Since the MSS options are an
announcement rather then a negotionation, it is likely the receiver will
have a different and possibly bigger MSS than allowed by our side.
This allow potentially for different a MSS in the receive and transmit
path.

Directly using the received MSS could cause problems when our MSS is only
allowed to be small. At transmission, for that reason take the minimum of
the received MSS and our desired MSS to find a value compatible to both
sides of the link.

Rename the function to net_tcp_get_recv_mss to net_tcp_get_supported_mss
to better reflect its function in the new situation.

Signed-off-by: Sjors Hettinga <s.a.hettinga@gmail.com>