selftests/net: Print TCP flags in more common format

selftests/net: Print TCP flags in more common format

Before:
># 13145[lib/ftrace-tcp.c:427] trace event filter tcp_ao_key_not_found [2001:db8:1::1:-1 => 2001:db8:254::1:7010, L3index 0, flags: !FS!R!P!., keyid: 100, rnext: 100, maclen: -1, sne: -1] = 1

After:
># 13487[lib/ftrace-tcp.c:427] trace event filter tcp_ao_key_not_found [2001:db8:1::1:-1 => 2001:db8:254::1:7010, L3index 0, flags: S, keyid: 100, rnext: 100, maclen: -1, sne: -1] = 1

For the history, I think the initial format was to emphasize the absence
of flags as well as their presence (!R meant no RST flag). But looking
again, it's just unreadable and hard to understand.
Make it the standard/expected one.

Signed-off-by: Dmitry Safonov <0x7f454c46@gmail.com>
Link: https://patch.msgid.link/20250319-tcp-ao-selftests-polling-v2-1-da48040153d1@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>