svcrdma: Populate tail iovec when receiving

So that NFS WRITE payloads can eventually be placed directly into a
file's page cache, enable the RPC-over-RDMA transport to present
these payloads in the xdr_buf's page list, while placing trailing
content (such as a GETATTR operation) in the xdr_buf's tail.

After this change, the RPC-over-RDMA's "copy tail" hack, added by
commit a97c331f

 ("svcrdma: Handle additional inline content"),
is no longer needed and can be removed.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>