SUNRPC xptrdma: simplify build configuration

Trond and Bruce,

This is a patch for 2.6.25. This is the same version that was sent out
on December 12 for review (no comments to date).

To simplify the RPC/RDMA client and server build configuration, make
SUNRPC_XPRT_RDMA a hidden config option that continues to depend on
SUNRPC and INFINIBAND. The value of SUNRPC_XPRT_RDMA will be:

 - N if either SUNRPC or INFINIBAND are N
 - M if both SUNRPC and INFINIBAND are on (M or Y) and at least one is M
 - Y if both SUNRPC and INFINIBAND are Y

In 2.6.25, all of the RPC/RDMA related files are grouped in
net/sunrpc/xprtrdma and the net/sunrpc/xprtrdma/Makefile builds both
the client and server RPC/RDMA support using this config option.

Signed-off-by: James Lentini <jlentini@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>