x86/mm/kcore: Add vsyscall page to /proc/kcore conditionally

The vsyscall page should be visible only if vsyscall=emulate/native when dumping /proc/kcore.

Signed-off-by: Jia Zhang <zhang.jia@linux.alibaba.com>
Reviewed-by: Jiri Olsa <jolsa@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: jolsa@redhat.com
Link: http://lkml.kernel.org/r/1518446694-21124-3-git-send-email-zhang.jia@linux.alibaba.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/mm/init_64.c

@@ -1193,7 +1193,8 @@ void __init mem_init(void)
 	register_page_bootmem_info();
 
 	/* Register memory areas for /proc/kcore */
-	kclist_add(&kcore_vsyscall, (void *)VSYSCALL_ADDR, PAGE_SIZE, KCORE_USER);
+	if (get_gate_vma(&init_mm))
+		kclist_add(&kcore_vsyscall, (void *)VSYSCALL_ADDR, PAGE_SIZE, KCORE_USER);
 
 	mem_init_print_info(NULL);
 }