MINOR: stick-table: make stktable_fetch_key() indicate why it failed

stktable_fetch_key() does not indicate whether it returns NULL because
the input sample was not found or because it's unstable. It causes trouble
with track-sc* rules. Just like with sample_fetch_string(), we want it to
be able to give more information to the caller about what it found. Thus,
now we use the pointer to a sample passed by the caller, and fill it with
the information we have about the sample. That way, even if we return NULL,
the caller has the ability to check whether a sample was found and if it is
still changing or not.
diff --git a/include/proto/stick_table.h b/include/proto/stick_table.h
index 0c26fbe..57ca223 100644
--- a/include/proto/stick_table.h
+++ b/include/proto/stick_table.h
@@ -48,7 +48,7 @@
 struct stksess *stktable_update_key(struct stktable *table, struct stktable_key *key);
 struct stktable_key *stktable_fetch_key(struct stktable *t, struct proxy *px,
 				        struct session *l4, void *l7, unsigned int opt,
-				        struct sample_expr *expr);
+				        struct sample_expr *expr, struct sample *smp);
 int stktable_compatible_sample(struct sample_expr *expr, unsigned long table_type);
 int stktable_get_data_type(char *name);
 struct proxy *find_stktable(const char *name);