Unbreak the build on architectures where size_t isn't synonymous with uintmax_t

I should have used %zu instead of %ju with size_t types.

MFC after: 1 month
MFC with: r343362, r343365
Approved by: emaste (mentor; implicit)
Reviewed by: asomers
Pointyhat to: ngie
Submitted by: asomers
Differential Revision: https://reviews.freebsd.org/D18935