btrfs: use fs_holder_ops for all opened devices

Since we have btrfs_fs_info::sb (struct super_block) as our block device
holder, we can safely use fs_holder_ops for all of our block devices.

This enables freezing/thawing the filesystem from the underlying block
devices.

This may enhance hibernation/suspend support since previously
freezing/thawing a block device managed by btrfs won't do anything btrfs
specific, but only syncing the block device.

Thus before this change, freezing the underlying block devices won't
prevent future writes into the filesystem, thus may cause problems for
hibernation/suspend when btrfs is involved.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>