x86/kexec: Make variable static and config dependent

The following sparse warning is emitted:

  arch/x86/kernel/crash.c:59:15:
  warning: symbol 'crash_zero_bytes' was not declared. Should it be static?

The variable is only used in this compilation unit, but it is also only
used when CONFIG_KEXEC_FILE is enabled. Just making it static would result
in a 'defined but not used' warning for CONFIG_KEXEC_FILE=n.

Make it static and move it into the existing CONFIG_KEXEC_FILE section.

[ tglx: Massaged changelog and moved it into the existing ifdef ]

Fixes: dd5f7260

 ("kexec: support for kexec on panic using new system call")
Signed-off-by: Tiezhu Yang <kernelpatch@126.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Dave Young <dyoung@redhat.com>
Cc: bp@alien8.de
Cc: hpa@zytor.com
Cc: kexec@lists.infradead.org
Cc: vgoyal@redhat.com
Cc: Vivek Goyal <vgoyal@redhat.com>
Link: https://lkml.kernel.org/r/117ef0c6.3d30.16b87c9cfbf.Coremail.kernelpatch@126.com