DESC
SKAS3: add locking around ->mm for PTRACE_SWITCH_MM
EDESC
From: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>

I just discovered that task_lock() and task_unlock() is used to protect
task_struct->mm... so this patch makes sure it is used for this purpose in
SKAS code.

Seeing this comment:

/*
 * Protects ->fs, ->files, ->mm, ->ptrace, ->group_info, ->comm and
 * synchronises with wait4().
 *
 * Nests both inside and outside of read_lock(&tasklist_lock).
 * It must not be nested with write_lock_irq(&tasklist_lock),
 * neither inside nor outside.
 */
static inline void task_lock(struct task_struct *p)

I've verified to some extent that it is called even without holding
tasklist_lock (for instance during execve).