AZL-99588

In the Linux kernel, the following vulnerability has been resolved:

tcp: clamp route advmss to TCPMINMSS

tcpselectinitial_window() assumes that callers never pass an MSS smaller than 1, but route-derived advmss values can violate that assumption.

A too-small explicit RTAXADVMSS is one way to get there, but it is not the only one. The same divide-by-zero can also be reached through the "default advmss" path when RTAXADVMSS is left at 0 and the effective advmss is later driven down by route MTU and minadvmss.

Introduce a tcpdstadvmss() helper that clamps route advmss to TCPMINMSS before TCP consumes it, and use it in the TCP paths that derive advmss from dst metrics. This keeps the effective MSS from dropping to zero before tcpselectinitial_window() rounds the receive window.