commit 88de6af2 upstream.

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

Fixes: fb7a0b9a ("nfs41: New backchannel helper routines")
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
[lizf: Backported to 3.4: adjust context]
Signed-off-by: Zefan Li <lizefan@huawei.com>