procfs: Use generic_file_llseek in /proc/vmcore

/proc/vmcore has no llseek and then falls down to use default_llseek.
This is racy against read_vmcore() that directly manipulates fpos
but it doesn't hold the bkl there so using it in llseek doesn't
protect anything.

Let's use generic_file_llseek() instead.

Signed-off-by: Frederic Weisbecker <[email protected]>
Acked-by: Arnd Bergmann <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: John Kacur <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Cc: Al Viro <[email protected]>

fs/proc/vmcore.c

@@ -162,6 +162,7 @@ static ssize_t read_vmcore(struct file *file, char __user *buffer,
 
 static const struct file_operations proc_vmcore_operations = {
 	.read		= read_vmcore,
+	.llseek		= generic_file_llseek,
 };
 
 static struct vmcore* __init get_new_element(void)