bcachefs: Fix setting of downgrade recovery passes/errors

bch2_check_version_downgrade() was setting c->sb.version, which
bch2_sb_set_downgrade() expects to be at the previous version; and it
shouldn't even have been set directly because c->sb.version is updated
by write_super().

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