You paid off the loan with an offset distribution in December, effectively a distribution paid to you that is immediately turned around to pay off the loan, so the loan is done. What you are permitted to do is roll over an amount equal to the amount of the offset distribution and you have until the due date of your 2018 tax return, including extensions, to complete the rollover if you can come up with the money. That's what the code M means. You'll need to tell the IRA custodian that it's a rollover of an offset distribution because the IRA custodian needs to report it with code PO in box 13c of the 2019 Form 5498 from this IRA.
