SUNRPC: Revert cc93ce95 ("svcrdma: Retain the page backing rq_res.head[0].iov_base")

Pre-requisite for releasing pages in the send completion handler.
Reverted by hand: patch -R would not apply cleanly.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>