clntrdma.h: Add a field to rpcrdma_xprt

Add a maxsegs field to rpcrdma_xprt, which is used to set
the number of segments allowed for a chunk based on device
attributes.

This commit should not affect non-RDMA behaviour.

MFC after:	3 months
Fixes:	884ee8d6c9 ("nfscl: Add some glue for client side NFS over RDMA")

sys/rpc/clntrdma.h

@@ -27,6 +27,7 @@ struct rpcrdma_xprt {
 	uint32_t	maxbck;
 	uint32_t	maxio;
 	uint32_t	maxsge;
+	uint32_t	maxsegs;
 	uint32_t	use_bounce;
 	void		*ep;
 };