mm: mmap: annotate vm_lock_anon_vma locking properly for lockdep

Commit 5a505085 ("mm/rmap: Convert the struct anon_vma::mutex to an
rwsem") turned anon_vma mutex to rwsem.

However, the properly annotated nested locking in mm_take_all_locks()
has been converted from

	mutex_lock_nest_lock(&anon_vma->root->mutex, &mm->mmap_sem);

to

	down_write(&anon_vma->root->rwsem);

which is incomplete, and causes the false positive report from lockdep
below.

Annotate the fact that mmap_sem is used as an outter lock to serialize
taking of all the anon_vma rwsems at once no matter the order, using the
down_write_nest_lock() primitive.

This patch fixes this lockdep report:

 =============================================
 [ INFO: possible recursive locking detected ]
 3.8.0-rc2-00036-g5f73896 #171 Not tainted
 ---------------------------------------------
 qemu-kvm/2315 is trying to acquire lock:
  (&anon_vma->rwsem){+.+...}, at: mm_take_all_locks+0x149/0x1b0

 but task is already holding lock:
  (&anon_vma->rwsem){+.+...}, at: mm_take_all_locks+0x149/0x1b0

 other info that might help us debug this:
  Possible unsafe locking scenario:

        CPU0
        ----
   lock(&anon_vma->rwsem);
   lock(&anon_vma->rwsem);

  *** DEADLOCK ***

  May be due to missing lock nesting notation

 4 locks held by qemu-kvm/2315:
  #0:  (&mm->mmap_sem){++++++}, at: do_mmu_notifier_register+0xfc/0x170
  #1:  (mm_all_locks_mutex){+.+...}, at: mm_take_all_locks+0x36/0x1b0
  #2:  (&mapping->i_mmap_mutex){+.+...}, at: mm_take_all_locks+0xc9/0x1b0
  #3:  (&anon_vma->rwsem){+.+...}, at: mm_take_all_locks+0x149/0x1b0

 stack backtrace:
 Pid: 2315, comm: qemu-kvm Not tainted 3.8.0-rc2-00036-g5f73896 #171
 Call Trace:
   print_deadlock_bug+0xf2/0x100
   validate_chain+0x4f6/0x720
   __lock_acquire+0x359/0x580
   lock_acquire+0x121/0x190
   down_write+0x3f/0x70
   mm_take_all_locks+0x149/0x1b0
   do_mmu_notifier_register+0x68/0x170
   mmu_notifier_register+0xe/0x10
   kvm_create_vm+0x22b/0x330 [kvm]
   kvm_dev_ioctl+0xf8/0x1a0 [kvm]
   do_vfs_ioctl+0x9d/0x350
   sys_ioctl+0x91/0xb0
   system_call_fastpath+0x16/0x1b

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Cc: Rik van Riel <riel@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mel Gorman <mel@csn.ul.ie>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>