Change sunrpc to use more list.h lists

The sunrpc client code uses home-grown doubly linked
lists to group
   - idle server threads
   - pending server sockets
   - waiting rpc tasks
   - all rpc tasks.

This patch converts all of these lists to <linux/list.h> lists and
also makes the list of all server sockets for a particular server into
a list.h list instead of a single-link list.

Possibly the least obvious change is replacing RPC_INIT_WAITQ
with RPC_WAITQ and INIT_RPC_WAITQ.  These follow the model of
LIST_HEAD and INIT_LIST_HEAD defined in list.h and are needed
to initialise the list_head in the rpc_waitq properly.