arm64: fix the flush_icache_range arguments in machine_kexec

The second argument is the end "pointer", not the length.

Fixes: d28f6df1

 ("arm64/kexec: Add core kexec support")
Cc: <stable@vger.kernel.org> # 4.8.x-
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>