btr_push_update_extern_fields(): Add a parameter for the original number
of fields in the record before btr_cur_trim(). Assume that this function
will only be called for the clustered index, which is the only index
that can contain off-page columns.

trx_undo_prev_version_build(), btr_cur_pessimistic_update():
Only invoke btr_push_update_extern_fields() for the clustered index.