SUNRPC xprtrdma: fix XDR tail buf marshalling for all ops

rpcrdma_convert_iovs is passed an xdr_buf representing either an RPC
request or an RPC reply. In the case of a request, several
calculations and tests involving pos are unnecessary. In the case of a
reply, several calculations and tests involving pos are incorrect (the
code tests pos against the reply xdr buf's len field, which is always
0 at the time rpcrdma_convert_iovs is executed). This change removes
the incorrect/unnecessary calculations and tests involving pos.

This fixes an observed problem when reading certain file sizes over
NFS/RDMA.

Signed-off-by: Tom Tucker <tom@opengridcomputing.com>
Signed-off-by: Tom Talpey <talpey@netapp.com>
Signed-off-by: James Lentini <jlentini@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>