When preparing your 2018 tax return you'll indicate as rolled over whatever amount of the code-M distribution you were able to roll over. Any portion not rolled over would be a cash-out.
If you anticipate rolling over an offset distribution, you should request a filing extension by April 15, 2019 if you do not complete the rollover by that date, then file your tax return before October 15, 2019 after rolling over as much as you intend to roll over.
