The compiler can't see that the definitions inside the .h file aren't meant to be
public symbols. So in a hypothetical program which uses the Kernel API, each(\!)
compilation unit that includes FB.h would define those fb_get_size_in_bytes symbols.
If that happens twice or more times, that would cause linker errors.
Since the functions are very short, inlining them seems like a good idea.
Also, using FB.h should be possible even if the containing compilation unit
doesn't already define size_t, so I added that header (stddef), too.