fork: Move and describe why the code examines PIDNS_ADDING

Normally this would be something that would be handled by handling
signals that are sent to a group of processes but in this case the
forking process is not a member of the group being signaled.  Thus
special code is needed to prevent a race with pid namespaces exiting,
and fork adding new processes within them.

Move this test up before the signal restart just in case signals are
also pending.  Fatal conditions should take presedence over restarts.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>