btrfs: implement remove_bdev and shutdown super operation callbacks

btrfs: implement remove_bdev and shutdown super operation callbacks

For the ->remove_bdev() callback, btrfs will:

- Mark the target device as missing

- Go degraded if the fs can afford it

- Return error other wise
  Thus falls back to the shutdown callback

For the ->shutdown callback, btrfs will:

- Set the SHUTDOWN flag
  Which will reject all new incoming operations, and make all writeback
  to fail.

  The behavior is the same as the NOLOGFLUSH behavior.

To support the lookup from bdev to a btrfs_device,
btrfs_dev_lookup_args is enhanced to have a new @devt member.
If set, we should be able to use that @devt member to uniquely locating a
btrfs device.

I know the shutdown can be a little overkilled, if one has a RAID1
metadata and RAID0 data, in that case one can still read data with 50%
chance to got some good data.

But a filesystem returning -EIO for half of the time is not really
considered usable.
Further it can also be as bad as the only device went missing for a single
device btrfs.

So here we go safe other than sorry when handling missing device.

And the remove_bdev callback will be hidden behind experimental features
for now, the reasons are:

- There are not enough btrfs specific bdev removal test cases
  The existing test cases are all removing the only device, thus only
  exercises the ->shutdown() behavior.

- Not yet determined what's the expected behavior
  Although the current auto-degrade behavior is no worse than the old
  behavior, it may not always be what the end users want.

  Before there is a concrete interface, better hide the new feature
  from end users.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <asj@kernel.org>
Tested-by: Anand Jain <asj@kernel.org>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>