bpf: selftests: Restore netns after each test

bpf: selftests: Restore netns after each test

[ Upstream commit 811d7e375d08312dba23f3b6bf7e58ec14aa5dcb ]

It is common for networking tests creating its netns and making its own
setting under this new netns (e.g. changing tcp sysctl).  If the test
forgot to restore to the original netns, it would affect the
result of other tests.

This patch saves the original netns at the beginning and then restores it
after every test.  Since the restore "setns()" is not expensive, it does it
on all tests without tracking if a test has created a new netns or not.

The new restore_netns() could also be done in test__end_subtest() such
that each subtest will get an automatic netns reset.  However,
the individual test would lose flexibility to have total control
on netns for its own subtests.  In some cases, forcing a test to do
unnecessary netns re-configure for each subtest is time consuming.
e.g. In my vm, forcing netns re-configure on each subtest in sk_assign.c
increased the runtime from 1s to 8s.  On top of that,  test_progs.c
is also doing per-test (instead of per-subtest) cleanup for cgroup.
Thus, this patch also does per-test restore_netns().  The only existing
per-subtest cleanup is reset_affinity() and no test is depending on this.
Thus, it is removed from test__end_subtest() to give a consistent
expectation to the individual tests.  test_progs.c only ensures
any affinity/netns/cgroup change made by an earlier test does not
affect the following tests.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200702004858.2103728-1-kafai@fb.com
Signed-off-by: Sasha Levin <sashal@kernel.org>