SUNRPC: Fix a memory leak in the backchannel code

SUNRPC: Fix a memory leak in the backchannel code

commit 88de6af24f2b48b06c514d3c3d0a8f22fafe30bd upstream.

req->rq_private_buf isn't initialised when xprt_setup_backchannel calls
xprt_free_allocation.

Fixes: fb7a0b9addbdb ("nfs41: New backchannel helper routines")
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
(cherry picked from commit bd031759bcc0865938ecc52846a3f22f2a26ad8e)

Signed-off-by: Willy Tarreau <w@1wt.eu>