xprtrdma: Clean up rpcrdma_prepare_readch()

Since commit 9ed5af26

 ("SUNRPC: Clean up the handling of page
padding in rpc_prepare_reply_pages()") [Dec 2020] the NFS client
passes payload data to the transport with the padding in xdr->pages
instead of in the send buffer's tail kvec. There's no need for the
extra logic to advance the base of the tail kvec because the upper
layer no longer places XDR padding there.

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