selftests: rds: Fix tcpdump segfault in rds selftests

selftests: rds: Fix tcpdump segfault in rds selftests

net/rds/test.py sees a segfault in tcpdump when executed through the
ksft runner.

[   21.903713] tcpdump[1469]: segfault at 0 ip 000072100e99126d
sp 00007ffccf740fd0 error 4
[   21.903721]  in libc.so.6[16a26d,7798b149a000+188000]
[   21.905074]  in libc.so.6[16a26d,72100e84f000+188000] likely on
CPU 5 (core 5, socket 0)
[   21.905084] Code: 00 0f 85 a0 00 00 00 48 83 c4 38 89 d8 5b 41 5c
41 5d 41 5e 41 5f 5d c3 0f 1f 44 00 00 48 8b 05 91 8b 09 00 8b 4d ac
64 89 08 <41> 0f b6 07 83 e8 2b a8 fd 0f 84 54 ff ff ff 49 8b 36 4c 89
ff e8
[   21.906760]  likely on CPU 9 (core 9, socket 0)
[   21.913469] Code: 00 0f 85 a0 00 00 00 48 83 c4 38 89 d8 5b 41 5c 41
5d 41 5e 41 5f 5d c3 0f 1f 44 00 00 48 8b 05 91 8b 09 00 8b 4d ac 64 89
08 <41> 0f b6 07 83 e8 2b a8 fd 0f 84 54 ff ff ff 49 8b 36 4c 89 ff e8

The os.fork() call creates extra complexity because it forks the entire
process including the python interpreter.  ip() then calls cmd() which
creates a subprocess.Popen.  We can avoid the extra layering by simply
calling subprocess.Popen directly. Track the process handles directly
and terminate them at cleanup rather than relying on killall. Further
tcpdump's -Z flag attempts to change savefile ownership, which is not
supported by the 9p protocol.  Fix this by writing pcap captures to
"/tmp" during the test and move them to the log directory after tcpdump
exits.

Signed-off-by: Allison Henderson <achender@kernel.org>
Link: https://patch.msgid.link/20260308055835.1338257-4-achender@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>