sunrpc/cache: change cache_defer_req to return -ve error, not boolean.

As "cache_defer_req" does not sound like a predicate, having it return
a boolean value can be confusing.  It is more consistent to return
0 for success and negative for error.

Exactly what error code to return is not important as we don't
differentiate between reasons why the request wasn't deferred,
we only care about whether it was deferred or not.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>