sunrpc/cache: change deferred-request hash table to use hlist.

Being a hash table, hlist is the best option.

There is currently some ugliness were we treat "->next == NULL" as
a special case to avoid having to initialise the whole array.
This change nicely gets rid of that case.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>