md: Unify mddev destruction paths

Previously, mddev_put() had a couple different paths for freeing a
mddev, due to the fact that the kobject wasn't initialized when the
mddev was first allocated. If we move the kobject_init() to when it's
first allocated and just use kobject_add() later, we can clean all this
up.

This also removes a hack in mddev_put() to avoid freeing biosets under a
spinlock, which involved copying biosets on the stack after the reset
bioset_init() changes.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>