bcachefs: Implement blk_holder_ops

bcachefs: Implement blk_holder_ops

We can't use the standard fs_holder_ops because they're meant for single
device filesystems - fs_bdev_mark_dead() in particular - and they assume
that the blk_holder is the super_block, which also doesn't work for a
multi device filesystem.

These generally follow the standard fs_holder_ops; the
locking/refcounting is a bit simplified because c->ro_ref suffices, and
bch2_fs_bdev_mark_dead() is not necessarily shutting down the entire
filesystem.

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