dm: fix thaw_bdev

This patch fixes a bd_mount_sem counter corruption bug in device-mapper.

thaw_bdev() should be called only when freeze_bdev() was called for the
device.
Otherwise, thaw_bdev() will up bd_mount_sem and corrupt the semaphore counter.
struct block_device with the corrupted semaphore may remain in slab cache
and be reused later.

Attached patch will fix it by calling unlock_fs() instead.
unlock_fs() will determine whether it should call thaw_bdev()
by checking the device is frozen or not.

Easy reproducer is:
  #!/bin/sh
  while [ 1 ]; do
     dmsetup --notable create a
     dmsetup --nolockfs suspend a
     dmsetup remove a
  done

It's not easy to see the effect of corrupted semaphore.
So I have tested with putting printk below in bdev_alloc_inode():
        if (atomic_read(&ei->bdev.bd_mount_sem.count) != 1)
                printk(KERN_DEBUG "Incorrect semaphore count = %d (%p)\n",
                        atomic_read(&ei->bdev.bd_mount_sem.count),
                        &ei->bdev);

Without the patch, I saw something like:
 Incorrect semaphore count = 17 (f2ab91c0)

With the patch, the message didn't appear.

The bug was introduced in 2.6.16 with this bug fix:

commit d9dde59b


Date:   Fri Feb 24 13:04:24 2006 -0800

    [PATCH] dm: missing bdput/thaw_bdev at removal

    Need to unfreeze and release bdev otherwise the bdev inode with
    inconsistent state is reused later and cause problem.

and backported to 2.6.15.5.

It occurs only in free_dev(), which is called only when the dm device is
removed.  The buggy code is executed only if md->suspended_bdev is
non-NULL and that can happen only when the device was suspended without
noflush.

Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Cc: stable@kernel.org