selftest: Add test for SO_INCOMING_CPU.

Some highly optimised applications use SO_INCOMING_CPU to make them
efficient, but they didn't test if it's working correctly by getsockopt()
to avoid slowing down.  As a result, no one noticed it had been broken
for years, so it's a good time to add a test to catch future regression.

The test does

  1) Create $(nproc) TCP listeners associated with each CPU.

  2) Create 32 child sockets for each listener by calling
     sched_setaffinity() for each CPU.

  3) Check if accept()ed sockets' sk_incoming_cpu matches
     listener's one.

If we see -EAGAIN, SO_INCOMING_CPU is broken.  However, we might not see
any error even if broken; the kernel could miraculously distribute all SYN
to correct listeners.  Not to let that happen, we must increase the number
of clients and CPUs to some extent, so the test requires $(nproc) >= 2 and
creates 64 sockets at least.

Test:
  $ nproc
  96
  $ ./so_incoming_cpu

Before the previous patch:

  # Starting 12 tests from 5 test cases.
  #  RUN           so_incoming_cpu.before_reuseport.test1 ...
  # so_incoming_cpu.c:191:test1:Expected cpu (5) == i (0)
  # test1: Test terminated by assertion
  #          FAIL  so_incoming_cpu.before_reuseport.test1
  not ok 1 so_incoming_cpu.before_reuseport.test1
  ...
  # FAILED: 0 / 12 tests passed.
  # Totals: pass:0 fail:12 xfail:0 xpass:0 skip:0 error:0

After:

  # Starting 12 tests from 5 test cases.
  #  RUN           so_incoming_cpu.before_reuseport.test1 ...
  # so_incoming_cpu.c:199:test1:SO_INCOMING_CPU is very likely to be working correctly with 3072 sockets.
  #            OK  so_incoming_cpu.before_reuseport.test1
  ok 1 so_incoming_cpu.before_reuseport.test1
  ...
  # PASSED: 12 / 12 tests passed.
  # Totals: pass:12 fail:0 xfail:0 xpass:0 skip:0 error:0

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>