fix: block: merge struct block_device and struct hd_struct (v5.11)

See upstream commit :

  commit 0d02129e76edf91cf04fabf1efbc3a9a1f1d729a
  Author: Christoph Hellwig <hch@lst.de>
  Date:   Fri Nov 27 16:43:51 2020 +0100

    block: merge struct block_device and struct hd_struct

    Instead of having two structures that represent each block device with
    different life time rules, merge them into a single one.  This also
    greatly simplifies the reference counting rules, as we can use the inode
    reference count as the main reference count for the new struct
    block_device, with the device model reference front ending it for device
    model interaction.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Iaf663c689edb23905aeedc182d2aa268981f13dd