md: rewrite md_setup_drive to avoid ioctls

md_setup_drive knows it works with md devices, so it is rather pointless
to open a file descriptor and issue ioctls.  Just call directly into the
relevant low-level md routines after getting a handle to the device using
blkdev_get_by_dev instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: NeilBrown <neilb@suse.de>
Acked-by: Song Liu <song@kernel.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>