xprtrdma: Update the GFP flags used in xprt_rdma_allocate()

Reflect the more conservative approach used in the socket transport's
version of this transport method. An RPC buffer allocation should
avoid forcing not just FS activity, but any I/O.

In particular, two recent changes missed updating xprtrdma:

 - Commit c6c8fe79 ("net, sunrpc: suppress allocation warning ...")
 - Commit a564b8f0 ("nfs: enable swap on NFS")

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