SUNRPC: Trap RDMA segment overflows

Prevent svc_rdma_build_writes() from walking off the end of a Write
chunk's segment array. Caught with KASAN.

The test that this fix replaces is invalid, and might have been left
over from an earlier prototype of the PCL work.

Fixes: 7a1cbfa1

 ("svcrdma: Use parsed chunk lists to construct RDMA Writes")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>