logging: Copy the filter_fn and fix the IMSI filter in OpenBSC

The filter_fn has not been copied into the new structure breaking
the imsi and other filters in OpenBSC. Looking at the code we should
also introduce a callback for the reset of the context so we could
use subscr_get/subscr_put on the subscriber structure.
diff --git a/src/logging.c b/src/logging.c
index 214ad2a..2e3a80a 100644
--- a/src/logging.c
+++ b/src/logging.c
@@ -768,6 +768,7 @@
 	if (!osmo_log_info)
 		return -ENOMEM;
 
+	osmo_log_info->filter_fn = inf->filter_fn;
 	osmo_log_info->num_cat_user = inf->num_cat;
 	/* total number = number of user cat + library cat */
 	osmo_log_info->num_cat = inf->num_cat + ARRAY_SIZE(internal_cat);