m68k: rename global variable vmalloc_end to m68k_vmalloc_end

On SUN3, m68k defines macro VMALLOC_END as unsigned long variable
vmalloc_end which is adjusted from mmu_emu_init().  This becomes
problematic if a local variables vmalloc_end is defined in some
function (not very unlikely) and VMALLOC_END is used in the function -
the function thinks its referencing the global VMALLOC_END value but
would be referencing its own local vmalloc_end variable.

Rename the global variable to m68k_vmlloc_end which is much less
likely to be used as local variable name.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Roman Zippel <zippel@linux-m68k.org>