knfsd: move nfsv4 slab creation/destruction to module init/exit

We have some slabs that the nfs4 server uses to store state objects.
We're currently creating and destroying those slabs whenever the server
is brought up or down.  That seems excessive; may as well just do that
in module initialization and exit.

Also add some minor header cleanup.  (Thanks to Andrew Morton for that
and a compile fix.)

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>
Acked-by:  Neil Brown <neilb@suse.de>