block/cgroup: Split blkg_conf_prep()

block/cgroup: Split blkg_conf_prep()

Move the blkg_conf_open_bdev() call out of blkg_conf_prep() to make it
possible to add lock context annotations to blkg_conf_prep(). Change an
if-statement in blkg_conf_open_bdev() into a WARN_ON_ONCE() call. Export
blkg_conf_open_bdev() because it is called by the BFQ I/O scheduler and
the BFQ I/O scheduler may be built as a kernel module.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://patch.msgid.link/e6ea0387f413217c8561a0ca54ce7b846aa5c7c5.1780682325.git.bvanassche@acm.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>