xprtrdma: Recognize XDRBUF_SPARSE_PAGES

Commit 431f6eb3 ("SUNRPC: Add a label for RPC calls that require
allocation on receive") didn't update similar logic in rpc_rdma.c.
I don't think this is a bug, per-se; the commit just adds more
careful checking for broken upper layer behavior.

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