selftests/net: packetdrill: add tcp_dsack_mult.pkt

selftests/net: packetdrill: add tcp_dsack_mult.pkt

Test DSACK behavior with non contiguous ranges.

Without prior fix (tcp: fix tcp_ofo_queue() to avoid including
too much DUP SACK range) this would fail with:

tcp_dsack_mult.pkt:37: error handling packet: bad value outbound TCP option 5
script packet:  0.100682 . 1:1(0) ack 6001 <nop,nop,sack 1001:3001 7001:8001>
actual packet:  0.100679 . 1:1(0) ack 6001 win 1097 <nop,nop,sack 1001:6001 7001:8001>

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: xin.guo <guoxin0309@gmail.com>
Link: https://patch.msgid.link/20250626123420.1933835-3-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>