selftests: drv: net: fix test failure on ipv6 sys

selftests: drv: net: fix test failure on ipv6 sys

The `get_interface_info` call has ip version hard-coded which leads to
failures on an IPV6 system. The NetDrvEnv class already gathers
information about remote interface, so instead of fixing the local
implementation switch to using cfg.remote_ifname.

Before:
./drivers/net/ping.py
Traceback (most recent call last):
  File "/new_tests/./drivers/net/ping.py", line 217, in <module>
    main()
  File "/new_tests/./drivers/net/ping.py", line 204, in main
    get_interface_info(cfg)
  File "/new_tests/./drivers/net/ping.py", line 128, in get_interface_info
    raise KsftFailEx('Can not get remote interface')
net.lib.py.ksft.KsftFailEx: Can not get remote interface

After:
./drivers/net/ping.py
TAP version 13
1..6
ok 1 ping.test_default # SKIP Test requires IPv4 connectivity
ok 2 ping.test_xdp_generic_sb # SKIP Test requires IPv4 connectivity
ok 3 ping.test_xdp_generic_mb # SKIP Test requires IPv4 connectivity
ok 4 ping.test_xdp_native_sb # SKIP Test requires IPv4 connectivity
ok 5 ping.test_xdp_native_mb # SKIP Test requires IPv4 connectivity
ok 6 ping.test_xdp_offload # SKIP device does not support offloaded XDP
Totals: pass:0 fail:0 xfail:0 xpass:0 skip:6 error:0

Fixes: 75cc19c8ff89 ("selftests: drv-net: add xdp cases for ping.py")
Signed-off-by: Mohsin Bashir <mohsin.bashr@gmail.com>
Reviewed-by: David Wei <dw@davidwei.uk>
Link: https://patch.msgid.link/20250503013518.1722913-2-mohsin.bashr@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>