bcachefs: Ensure bch_sb_field_ext always exists

This makes bch_sb_field_ext more consistent with the rest of -o
nochanges - we don't want to be varying other codepaths based on -o
nochanges, since it's used for testing in dry run mode; also fixes some
potential null ptr derefs.

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