trx_undo_left(): Add const qualifiers.

trx_undo_page_report_insert(): Use exact trx_undo_left() limit.
Remove a duplicated trx_undo_left() check.

trx_undo_page_report_modify(): Eliminate the local variable len.
Document that no prefix for BLOBs needs to be stored in the undo log.
Lump two trx_undo_left() checks together.