sunrpc/cache: factor out cache_is_expired

This removes a tiny bit of code duplication, but more important
prepares for following patch which will perform the expiry check in
cache_lookup and the rest of the validity check in cache_check.

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