nfsd: simplify the delayed disposal list code

When queueing a dispose list to the appropriate "freeme" lists, it
pointlessly queues the objects one at a time to an intermediate list.

Remove a few helpers and just open code a list_move to make it more
clear and efficient. Better document the resulting functions with
kerneldoc comments.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>