net/tcp: Fix TCP port number reused on reboot

net/tcp: Fix TCP port number reused on reboot

GRUB's TCP stack assigns source ports for outgoing connections starting
at 21550 and increments sequentially by 1 (e.g., 21550, 21551, ...).
While this generally works, it can lead to failures if the system
reboots rapidly and reuses the same source port too soon.

This issue was observed on powerpc-ieee1275 platforms using CAS (Client
Architecture Support) reboot. In such cases, loading the initrd over
HTTP may fail with connection timeouts. Packet captures show the failed
connections are flagged as "TCP Port Number Reused" by Wireshark.

The root cause is that GRUB reuses the same port shortly after reboot,
while the server may still be tracking the previous connection in
TIME_WAIT. This can result in the server rejecting the connection
attempt or responding with a stale ACK or RST, leading to handshake
failure.

This patch fixes the issue by introducing a time based source port
selection strategy. Instead of always starting from port 21550, GRUB now
computes an initial base port based on the current RTC time, divided
into 5 minute windows. The purpose of this time based strategy is to
ensure that GRUB avoids reusing the same source port within a 5 minute
window, thereby preventing collisions with stale server side connection
tracking that could interfere with a new TCP handshake.

A step size of 8 ensures that the same port will not be reused across
reboots unless GRUB opens more than 8 TCP connections per second on
average, something that is highly unlikely. In typical usage, a GRUB
boot cycle lasts about 15 seconds and may open fewer than 100
connections total, well below the reuse threshold. This makes the
approach robust against short reboot intervals while keeping the logic
simple and deterministic.

Signed-off-by: Michael Chang <mchang@suse.com>
Reviewed-by: Sudhakar Kuppusamy <sudhakar@linux.ibm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>