btrfs: call bdev_fput() to reclaim the blk_holder immediately

As part of the preparation for btrfs blk_holder_ops, we want to ensure
the holder of a block device has a proper lifespan.

However btrfs is always using fput() to close a block device, which has
one problem:

- fput() is deferred
  Meaning we can have a block device with invalid (aka, freed) holder.

To avoid the problem and align the behavior to other code, just call
bdev_fput() instead.

There is some extra requirement on the locking, but that's all resolved
by previous patches and we should be safe to call bdev_fput().

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