fs: open block device after superblock creation

Currently get_tree_bdev and mount_bdev open the block device before
committing to allocating a super block. That creates problems for
restricting the number of writers to a device, and also leads to a
unusual and not very helpful holder (the fs_type).

Reorganize the super block code to first look whether the superblock for
a particular device does already exist and open the block device only if
it doesn't.

[hch: port to before the bdev_handle changes,
      duplicate the bdev read-only check from blkdev_get_by_path,
      extend the fsfree_mutex coverage to protect against freezes,
      fix an open bdev leak when the bdev is frozen,
      use the bdev local variable more,
      rename the s variable to sb to be more descriptive]
[brauner: remove references to mounts as they're mostly irrelevant]
[brauner & hch: fold fixes for romfs and cramfs for
                syzbot+2faac0423fdc9692822b@syzkaller.appspotmail.com]

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Message-Id: <20230724175145.201318-1-hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>