xprtrdma: Add a field of bit flags to struct rpcrdma_req

We have one boolean flag in rpcrdma_req today. I'd like to add more
flags, so convert that boolean to a bit flag.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>