md: factor out a mddev_find_locked helper from mddev_find

commit 8b57251f9a91f5e5a599de7549915d2d226cc3af upstream.

Factor out a self-contained helper to just lookup a mddev by the dev_t
"unit".

Cc: stable@vger.kernel.org
Reviewed-by: Heming Zhao <heming.zhao@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>