msgb: Allow size==headroom in msgb_alloc_headroom*()

Nothinh really forbids this case, it's totally fine allocating all space
of msgb as headroom. osmo-pcu actually does that in
gprs_rlcmac_ul_tbf::snd_ul_ud().

Related: OS#4029
Change-Id: Ibe05d08e3169a2603e891f76682a3b352a93ec7a
diff --git a/include/osmocom/core/msgb.h b/include/osmocom/core/msgb.h
index e05d37f..1833a6c 100644
--- a/include/osmocom/core/msgb.h
+++ b/include/osmocom/core/msgb.h
@@ -518,7 +518,7 @@
 static inline struct msgb *msgb_alloc_headroom_c(const void *ctx, int size, int headroom,
 						 const char *name)
 {
-	osmo_static_assert(size > headroom, headroom_bigger);
+	osmo_static_assert(size >= headroom, headroom_bigger);
 
 	struct msgb *msg = msgb_alloc_c(ctx, size, name);
 	if (msg)
@@ -540,7 +540,7 @@
 static inline struct msgb *msgb_alloc_headroom(int size, int headroom,
 						const char *name)
 {
-	osmo_static_assert(size > headroom, headroom_bigger);
+	osmo_static_assert(size >= headroom, headroom_bigger);
 
 	struct msgb *msg = msgb_alloc(size, name);
 	if (msg)