SUNRPC: Prevent mixed sign comparisons in rpcrdma_convert_iovs()

Keep the type of the buffer position the same during iovec conversion to
reduce the likelihood of unexpected results from comparisons and length
computations.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Cc: Thomas Talpey <Thomas.Talpey@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>