x86/kexec: crash_vmclear_local_vmcss needs __rcu

This removes the sparse warning:
arch/x86/kernel/crash.c:49:32: sparse: incompatible types in comparison expression (different address spaces)

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>