x86_64: flush_tlb_kernel_range() warning fix

mm/vmalloc.c: In function 'unmap_kernel_range':
mm/vmalloc.c:75: warning: unused variable 'start'

make it a C function so that the compiler thinks it used its arguments.

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>