bcachefs: Ensure devices are always correctly initialized

bcachefs: Ensure devices are always correctly initialized

We can't mark device superblocks or allocate journal on a device that
isn't online.

That means we may need to do this on every mount, because we may have
formatted a new filesystem and then done the first mount
(bch2_fs_initialize()) in degraded mode.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>