m68k: use some direct calls to ret_from_exception in entry code

The ret_from_excption code is referenced by its function name, or by a label
set at the start of its code. The non-MMU code can share some of this code
if we make direct calls to ret_from_exception instead of the associated label.
The effected function paths are: buserr, trap and ret_from_fork. So change
these to branch directly to ret_from_exception.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>