mm/vmalloc.c: export unmap_kernel_range()

zsmalloc needs exported unmap_kernel_range for building as a module.  See
https://lkml.org/lkml/2013/1/18/487

I didn't send a patch to make unmap_kernel_range exportable at that time
because zram was staging stuff and I thought VM function exporting for
staging stuff makes no sense.

Now zsmalloc was promoted.  If we can't build zsmalloc as module, it means
we can't build zram as module, either.  Additionally, buddy map_vm_area is
already exported so let's export unmap_kernel_range to help his buddy.

Signed-off-by: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Jerome Marchand <jmarchan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>