x86/mm/dump_pagetables: Use DEFINE_SHOW_ATTRIBUTE()

Use DEFINE_SHOW_ATTRIBUTE() instead of open coding it.

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: keescook@chromium.org
Cc: luto@kernel.org
Cc: peterz@infradead.org
Cc: bp@alien8.de
Cc: hpa@zytor.com
Link: https://lkml.kernel.org/r/20181119154334.18265-1-tiny.windzz@gmail.com