nfsd: register pernet ops last, unregister first

These pernet operations may depend on stuff set up or torn down in the
module init/exit functions.  And they may be called at any time in
between.  So it makes more sense for them to be the last to be
registered in the init function, and the first to be unregistered in the
exit function.

In particular, without this, the drc slab is being destroyed before all
the per-net drcs are shut down, resulting in an "Objects remaining in
nfsd_drc on __kmem_cache_shutdown()" warning in exit_nfsd.

Reported-by: Zhi Li <yieli@redhat.com>
Fixes: 3ba75830 "nfsd4: drc containerization"
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>