IPA: log OML/RSL link drop reason

There could multiple reason for OML or RSL link towards BTS to be
dropped: ctrl command, vty, new link etc. Introduce "reason" parameter
to corresponding functions and log it on link drop to simplify
troubleshooting issues with more complex setups.

Change-Id: I8c8d8132ba67c31e40dbecdfe2e09be08c744899
diff --git a/include/osmocom/bsc/ipaccess.h b/include/osmocom/bsc/ipaccess.h
index 692e795..3853270 100644
--- a/include/osmocom/bsc/ipaccess.h
+++ b/include/osmocom/bsc/ipaccess.h
@@ -30,9 +30,9 @@
 	uint8_t data[0];
 } __attribute__((packed));
 
-void ipaccess_drop_oml(struct gsm_bts *bts);
+void ipaccess_drop_oml(struct gsm_bts *bts, const char *reason);
 void ipaccess_drop_oml_deferred(struct gsm_bts *bts);
-void ipaccess_drop_rsl(struct gsm_bts_trx *trx);
+void ipaccess_drop_rsl(struct gsm_bts_trx *trx, const char *reason);
 
 struct sdp_header_item {
 	struct sdp_header_entry header_entry;