md: fix build failure for !MODULE

After merging the block tree, today's linux-next build (x86_64
allmodconfig) failed like this:

drivers/md/md.c:717:22: error: 'mddev_find' defined but not used [-Werror=unused-function]
  717 | static struct mddev *mddev_find(dev_t unit)
      |                      ^~~~~~~~~~
cc1: all warnings being treated as errors

Caused by commit

  4500d5c17910 ("md: simplify md_open")

Make mddev_find() available only for non-modular builds.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220721131132.070be166@canb.auug.org.au
Signed-off-by: Jens Axboe <axboe@kernel.dk>