sunrpc: remove unnecessary svc_xprt_put

The 'struct svc_deferred_req's on the xpt_deferred queue do not
own a reference to the owning xprt.  This is seen in svc_revisit
which is where things are added to this queue.  dr->xprt is set to
NULL and the reference to the xprt it put.

So when this list is cleaned up in svc_delete_xprt, we mustn't
put the reference.

Also, replace the 'for' with a 'while' which is arguably
simpler and more likely to compile efficiently.

Cc: Tom Tucker <tom@opengridcomputing.com>
Signed-off-by: NeilBrown <neilb@suse.de>
Cc: stable@kernel.org
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>