ksefltests: pidfd: Fix wait_states: Test terminated by timeout

0Day/LKP observed that the kselftest blocks forever since one of the
pidfd_wait doesn't terminate in 1 of 30 runs. After digging into
the source, we found that it blocks at:
ASSERT_EQ(sys_waitid(P_PIDFD, pidfd, &info, WCONTINUED, NULL), 0);

wait_states has below testing flow:
  CHILD                 PARENT
  ---------------+--------------
1 STOP itself
2                   WAIT for CHILD STOPPED
3                   SIGNAL CHILD to CONT
4 CONT
5 STOP itself
5'                  WAIT for CHILD CONT
6                   WAIT for CHILD STOPPED

The problem is that the kernel cannot ensure the order of 5 and 5', once
5 goes first, the test will fail.

we can reproduce it by:
$ while true; do make run_tests -C pidfd; done

Introduce a blocking read in child process to make sure the parent can
check its WCONTINUED.

CC: Philip Li <philip.li@intel.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>