Fastbuf: Get rid of ->is_fastbuf typechecking hack

Fastbuf: Get rid of ->is_fastbuf typechecking hack

... it breaks C99 aliasing rules and GCC rightfully warns about it.

If we ever want to do such type checking, we might use

	#define CAST(f) ((f)->is_fastbuf, (struct xxxx *)(f))

but I guess it is of little use anyway, so I am removing it.