sunrpc: extract some common sunrpc_cache code from nfsd

Rather can duplicating this idiom twice, put it in an inline function.
This reduces the usage of 'expiry_time' out side the sunrpc/cache.c
code and thus the impact of a change that is about to be made to that
field.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
diff --git a/include/linux/sunrpc/cache.h b/include/linux/sunrpc/cache.h
index 7bf3e84..0e1febf 100644
--- a/include/linux/sunrpc/cache.h
+++ b/include/linux/sunrpc/cache.h
@@ -228,4 +228,10 @@
 	return rv;
 }
 
+static inline void sunrpc_invalidate(struct cache_head *h,
+				     struct cache_detail *detail)
+{
+	h->expiry_time = get_seconds() - 1;
+	detail->nextcheck = get_seconds();
+}
 #endif /*  _LINUX_SUNRPC_CACHE_H_ */