[PATCH] adjust msgb api

Hi.

While looking at osmocom-bb compilation warnings I've noticed that there are plenty of those cause by signed-unsigned comparison with msgb-related functions. Attached is a trivial patch which fixes that by changing int -> uint16_t. For the sake of completeness I've also changed other functions to explicitly use uint16_t - this is used for length fields in "struct msgb" anyway.

Although technically it's API change I do not expect any sane code to break. If you still think it's potentially insecure, this could be applied during next api version bump alongside with gprs api change.