svcrpc: take lock on turning entry NEGATIVE in cache_check

We attempt to turn a cache entry negative in place.  But that entry may
already have been filled in by some other task since we last checked
whether it was valid, so we could be modifying an already-valid entry.
If nothing else there's a likely leak in such a case when the entry is
eventually put() and contents are not freed because it has
CACHE_NEGATIVE set.

So, take the cache_lock just as sunrpc_cache_update() does.

Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>