selftests/bpf: Use log_err in open_netns/close_netns

selftests/bpf: Use log_err in open_netns/close_netns

ASSERT helpers defined in test_progs.h shouldn't be used in public
functions like open_netns() and close_netns(). Since they depend on
test__fail() which defined in test_progs.c. Public functions may be
used not only in test_progs.c, but in other tests like test_sock_addr.c
in the next commit.

This patch uses log_err() to replace ASSERT helpers in open_netns()
and close_netns() in network_helpers.c to decouple dependencies, then
uses ASSERT_OK_PTR() to check the return values of all open_netns().

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/d1dad22b2ff4909af3f8bfd0667d046e235303cb.1713868264.git.tanggeliang@kylinos.cn
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>