pid_from_ns_wrapper: remove the loop

pid_from_ns_wrapper: remove the loop

If we clone a child which can't reply to us within the timeout, do
not keep looping, just return an error.  Commonize the function
superficially to make it look like pid_to_ns_wrapper().  Presumably
we can now merge these into one function, that's left for later.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>