selftests/net: Open /proc/thread-self in open_netns()

selftests/net: Open /proc/thread-self in open_netns()

It turns to be that open_netns() is called rarely from the child-thread
and more often from parent-thread. Yet, on initialization of kconfig
checks, either of threads may reach kconfig_lock mutex first.
VRF-related checks do create a temporary ksft-check VRF in
an unshare()'d namespace and than setns() back to the original.
As original was opened from "/proc/self/ns/net", it's valid for
thread-leader (parent), but it's invalid for the child, resulting
in the following failure on tests that check has_vrfs() support:
> # ok 54 TCP-AO required on socket + TCP-MD5 key: prefailed as expected: Key was rejected by service
> # not ok 55 # error 381[unsigned-md5.c:24] Failed to add a VRF: -17
> # not ok 56 # error 383[unsigned-md5.c:33] Failed to add a route to VRF: -22: Key was rejected by service
> not ok 1 selftests: net/tcp_ao: unsigned-md5_ipv6 # exit=1

Use "/proc/thread-self/ns/net" which is valid for any thread.

Signed-off-by: Dmitry Safonov <0x7f454c46@gmail.com>
Link: https://patch.msgid.link/20240823-tcp-ao-selftests-upd-6-12-v4-4-05623636fe8c@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>