sunrpc: clean up xdr_shrink_pagelen use of temporary pointer

char *p is used only as a shorthand for tail->iov_base + len in a nested
block.  Move it there.

Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>