Start using struct pid.

[ebiederm@xmission.com (Eric W. Biederman)]

In the last round of cleaning up the pid hash table a more
general struct pid was introduced, that can be referenced
counted.

With the more general struct pid most if not all places where
we store a pid_t we can now store a struct pid * and remove
the need for a hash table lookup, and avoid any possible problems
with pid roll over.

Looking forward to the pid namespaces struct pid * gives us
an absolute form a pid so we can compare and use them without
caring which pid namespace we are in.

This patchset introduces the infrastructure needed to use
struct pid instead of pid_t, and then it goes on to convert
two different kernel users that currently store a pid_t value.

There are a lot more places to go but this is enough to get the
basic idea. 

Before we can merge a pid namespace patch all of the kernel pid_t
users need to be examined.  Those that deal with user space processes
need to be converted to using a struct pid *.  Those that deal with
kernel processes need to converted to using the kthread api.  A rare
few that only use their current processes pid values get to be left
alone.

Eric