sgsn: Split gsm0408_gprs_force_reattach into 2 functions

This patch replaces gsm0408_gprs_force_reattach(msg, mmctx) by two
functions
  - gsm0408_gprs_force_reattach(mmctx)
  - gsm0408_gprs_force_reattach_oldmsg(msg)

The old function basically consists of the code of the two new
functions, where the code path selected depends on mmctx == NULL,
which is harder to maintain, less obvious to use, and not consistent
with many other SGSN functions.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/src/gprs/gprs_sgsn.c b/openbsc/src/gprs/gprs_sgsn.c
index 15f4753..c4ff3c2 100644
--- a/openbsc/src/gprs/gprs_sgsn.c
+++ b/openbsc/src/gprs/gprs_sgsn.c
@@ -448,6 +448,6 @@
 
 int sgsn_force_reattach_oldmsg(struct msgb *oldmsg)
 {
-	return gsm0408_gprs_force_reattach(oldmsg, NULL);
+	return gsm0408_gprs_force_reattach_oldmsg(oldmsg);
 }