SUNRPC: Clean up xdr_write_pages()

Make it more evident how xdr_write_pages() updates the tail buffer
by using the convention of naming the iov pointer variable "tail".
I spent more than a couple of hours chasing through code to
understand this, so someone is likely to find this useful later.

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