bcachefs: Improve opts.degraded

bcachefs: Improve opts.degraded

Kill 'opts.very_degraded', and make 'opts.degraded' a persistent option,
stored in the superblock.

It's now an enum, with available choices ask/yes/very/no.

"ask" mode will be handled by the mount helper, for prompting the user
(on a machine used interactively) for whether to do a degraded mount.

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