exofs: Convert to separately allocated bdi

Allocate struct backing_dev_info separately instead of embedding it
inside the superblock. This unifies handling of bdi among users.

CC: Boaz Harrosh <ooo@electrozaur.com>
CC: Benny Halevy <bhalevy@primarydata.com>
Acked-by: Boaz Harrosh <ooo@electrozaur.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>