btrfs: shrink the size of btrfs_device

btrfs: shrink the size of btrfs_device

There are two main causes of holes inside btrfs_device:

- The single bytes member of last_flush_error
  Not only it's a single byte member, but we never really care about the
  exact error number.

- The @devt member
  Which is placed between two u64 members.

Shrink the size of btrfs_device by:

- Use a single bit flag for flush error
  Use BTRFS_DEV_STATE_FLUSH_FAILED so that we no longer need that
  dedicated member.

- Move @devt to the hole after dev_stat_values[]

This reduces the size of btrfs_device from 528 to exact 512 bytes for
x86_64.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>