selftest: pidfd: Omit long and repeating outputs

An output message:

> # # waitpid WEXITSTATUS=0

will be printed for 30,000+ times in the `pidfd_test` selftest, which
does not seem ideal. This patch removes the print logic in the
`wait_for_pid` function, so each call to this function does not output
a line by default. Any existing call sites where the extra line might
be beneficial have been modified to include extra print statements
outside of the function calls.

Signed-off-by: Ziqi Zhao <astrajoan@yahoo.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>