msgb: use OSMO_ASSERT in msgb_alloc_headroom[_c]()

This patch is a preparation for the upcoming change making use of
the built-in static_assert(), which is available since C11.

When using built-in static_assert(), gcc v12.2.1 fails:

include/osmocom/core/msgb.h: In function 'msgb_alloc_headroom_c':
include/osmocom/core/msgb.h:532:33: error: expression in static assertion is not constant
  532 |         osmo_static_assert(size >= headroom, headroom_bigger);
include/osmocom/core/utils.h:86:24: note: in definition of macro 'osmo_static_assert'
   86 |         static_assert((exp), "(" #exp ") failed")
      |                        ^~~
include/osmocom/core/msgb.h: In function 'msgb_alloc_headroom':
include/osmocom/core/msgb.h:554:33: error: expression in static assertion is not constant
  554 |         osmo_static_assert(size >= headroom, headroom_bigger);
include/osmocom/core/utils.h:86:24: note: in definition of macro 'osmo_static_assert'
   86 |         static_assert((exp), "(" #exp ") failed")
      |                        ^~~

These are not really *static* assert()s, because they operate on the
user supplied arguments 'size' and 'headroom', which are not guaranteed
to be integer literals.  Neither they trigger compilation failures
as expected, nor do they abort at run-time.  They simply do nothing.

Change-Id: I17ef4f3283ce20a5b452b7874c826acfb02a0123
---
M include/osmocom/core/msgb.h
1 file changed, 34 insertions(+), 2 deletions(-)

diff --git a/include/osmocom/core/msgb.h b/include/osmocom/core/msgb.h
index 3fdb189..2529c0e 100644
--- a/include/osmocom/core/msgb.h
+++ b/include/osmocom/core/msgb.h
@@ -529,7 +529,7 @@
 static inline struct msgb *msgb_alloc_headroom_c(const void *ctx, uint16_t size, uint16_t headroom,
 						 const char *name)
 {
-	osmo_static_assert(size >= headroom, headroom_bigger);
+	OSMO_ASSERT(size >= headroom);
 
 	struct msgb *msg = msgb_alloc_c(ctx, size, name);
 	if (OSMO_LIKELY(msg))
@@ -551,7 +551,7 @@
 static inline struct msgb *msgb_alloc_headroom(uint16_t size, uint16_t headroom,
 						const char *name)
 {
-	osmo_static_assert(size >= headroom, headroom_bigger);
+	OSMO_ASSERT(size >= headroom);
 
 	struct msgb *msg = msgb_alloc(size, name);
 	if (OSMO_LIKELY(msg))

