SKAS3: add locking around ->mm for PTRACE_SWITCH_MM

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).

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Index: linux-2.6.14/arch/i386/kernel/ptrace.c
===================================================================
--- linux-2.6.14.orig/arch/i386/kernel/ptrace.c	2005-10-29 05:50:05.000000000 +0200
+++ linux-2.6.14/arch/i386/kernel/ptrace.c	2005-10-29 05:50:24.000000000 +0200
@@ -702,8 +702,10 @@
 		}
 
 		atomic_inc(&new->mm_users);
+		task_lock(child);
 		child->mm = new;
 		child->active_mm = new;
+		task_unlock(child);
 		mmput(old);
 		ret = 0;
 		break;