md: split mddev_find

commit 65aa97c4d2bfd76677c211b9d03ef05a98c6d68e upstream.

Split mddev_find into a simple mddev_find that just finds an existing
mddev by the unit number, and a more complicated mddev_find that deals
with find or allocating a mddev.

This turns out to fix this bug reported by Zhao Heming.

----------------------------- snip ------------------------------
commit d3374825 ("md: make devices disappear when they are no longer
needed.") introduced protection between mddev creating & removing. The
md_open shouldn't create mddev when all_mddevs list doesn't contain
mddev. With currently code logic, there will be very easy to trigger
soft lockup in non-preempt env.