fix: block: remove genhd.h (v5.18)

fix: block: remove genhd.h (v5.18)

See upstream commit :

  commit 322cbb50de711814c42fb088f6d31901502c711a
  Author: Christoph Hellwig <hch@lst.de>
  Date:   Mon Jan 24 10:39:13 2022 +0100

    block: remove genhd.h

    There is no good reason to keep genhd.h separate from the main blkdev.h
    header that includes it.  So fold the contents of genhd.h into blkdev.h
    and remove genhd.h entirely.

Change-Id: I7cf2aaa3a4c133320b95f2edde49f790f9515dbd
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>