xprtrdma: De-duplicate "allocate new, free old regbuf"

Clean up by providing an API to do this common task.

At this point, the difference between rpcrdma_get_sendbuf and
rpcrdma_get_recvbuf has become tiny. These can be collapsed into a
single helper.

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

net/sunrpc/xprtrdma/xprt_rdma.h

@@ -552,6 +552,8 @@ void rpcrdma_recv_buffer_put(struct rpcrdma_rep *);
 
 struct rpcrdma_regbuf *rpcrdma_alloc_regbuf(size_t, enum dma_data_direction,
 					    gfp_t);
+bool rpcrdma_regbuf_realloc(struct rpcrdma_regbuf *rb, size_t size,
+			    gfp_t flags);
 bool __rpcrdma_dma_map_regbuf(struct rpcrdma_ia *, struct rpcrdma_regbuf *);
 void rpcrdma_free_regbuf(struct rpcrdma_regbuf *);