osmo_io: Change parent of msghdr to iofd (instead of msg)

msg was made a parent of msghdr after discussion in change
I3a279b55a3adff96948120683c844e1508d0ba94

It turns out this violates some assumptions made in osmo_io,
specifically that the user read callback shall free msg, but we expect
msghdr to remain valid until after that callback returns.

In general I think it is cleaner to make iofd a parent of msghdr.

Change-Id: I41277190e3020cd8fa625bd57a743973e2a65c4b
---
M src/core/osmo_io.c
1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/src/core/osmo_io.c b/src/core/osmo_io.c
index e176099..c67787b 100644
--- a/src/core/osmo_io.c
+++ b/src/core/osmo_io.c
@@ -120,7 +120,7 @@
 		talloc_steal(iofd, msg);
 	}
 
-	hdr = talloc_zero(msg, struct iofd_msghdr);
+	hdr = talloc_zero(iofd, struct iofd_msghdr);
 	if (!hdr) {
 		if (free_msg)
 			talloc_free(msg);

