SUNRPC: Distinguish between the slot allocation list and receive queue

When storing a struct rpc_rqst on the slot allocation list, we currently
use the same field 'rq_list' as we use to store the request on the
receive queue. Since the structure is never on both lists at the same
time, this is OK.
However, for clarity, let's make that a union with different names for
the different lists so that we can more easily distinguish between
the two states.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>