m68k/sun3: Kill pte_unmap() warnings

Since commit 31c91132 ("mm: check the argument
of kunmap on architectures without highmem"), we get lots of warnings like

arch/m68k/kernel/sys_m68k.c:508: warning: passing argument 1 of ‘kunmap’ from incompatible pointer type

As m68k doesn't support highmem anyway, open code the calls to kmap() and
kunmap() (the latter is a no-op) to kill the warnings, like is done on most
other architectures without CONFIG_HIGHPTE.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Sam Creasey <sammy@sammy.net>