AppArmor: Fix sleep in invalid context from task_setrlimit

Affected kernels 2.6.36 - 3.0

AppArmor may do a GFP_KERNEL memory allocation with task_lock(tsk->group_leader);
held when called from security_task_setrlimit.  This will only occur when the
task's current policy has been replaced, and the task's creds have not been
updated before entering the LSM security_task_setrlimit() hook.

BUG: sleeping function called from invalid context at mm/slub.c:847
 in_atomic(): 1, irqs_disabled(): 0, pid: 1583, name: cupsd
 2 locks held by cupsd/1583:
  #0:  (tasklist_lock){.+.+.+}, at: [<ffffffff8104dafa>] do_prlimit+0x61/0x189
  #1:  (&(&p->alloc_lock)->rlock){+.+.+.}, at: [<ffffffff8104db2d>]
do_prlimit+0x94/0x189
 Pid: 1583, comm: cupsd Not tainted 3.0.0-rc2-git1 #7
 Call Trace:
  [<ffffffff8102ebf2>] __might_sleep+0x10d/0x112
  [<ffffffff810e6f46>] slab_pre_alloc_hook.isra.49+0x2d/0x33
  [<ffffffff810e7bc4>] kmem_cache_alloc+0x22/0x132
  [<ffffffff8105b6e6>] prepare_creds+0x35/0xe4
  [<ffffffff811c0675>] aa_re...