md: do not return existing mddevs from mddev_find_or_alloc

Instead of returning an existing mddev, just for it to be discarded
later directly return -EEXIST.  Rename the function to mddev_alloc now
that it doesn't find an existing mddev.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Song Liu <song@kernel.org>