tcp: do not increment BeyondWindow MIB for old seq

tcp: do not increment BeyondWindow MIB for old seq

The mentioned MIB is currently incremented even when a packet
with an old sequence number (i.e. a zero window probe) is received,
which is IMHO misleading.

Explicitly restrict such MIB increment at the relevant events.

Fixes: 6c758062c64d ("tcp: add LINUX_MIB_BEYOND_WINDOW")
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Link: https://patch.msgid.link/20d147292eb4b13b6535e0ad6f56be64d9c330d3.1753118029.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>