The lifetime of the data provided by a global variable is hard to track.
So move the data pointer into the nfs_priv structure and let rpc_req
return the data to its callers which in turn are responsible to free it.

The callers are changed to use a local variable accordingly. This makes
it plausible that the previous commit catched all usages of the global
data.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>