Add two new superblock fields. Since the main section of the superblock
is now fully, we have to add a new variable length section for them -
bch_sb_field_ext.

 - recovery_passes_requried: recovery passes that must be run on the
   next mount
 - errors_silent: errors that will be silently fixed

These are to improve upgrading and dwongrading: these fields won't be
cleared until after recovery successfully completes, so there won't be
any issues with crashing partway through an upgrade or a downgrade.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>