bcachefs: fix_errors option is now a proper enum

bcachefs: fix_errors option is now a proper enum

Before, it was parsed as a bool but internally it was really an enum:
this lets us pass in all the possible values.

But we special case the option parsing: no supplied value is parsed as
FSCK_FIX_yes, to match the previous behaviour.

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