makefs: Cast daddr_t to off_t before multiplication

Apparently some large-file systems out there, such as my powerpc64le
Linux box, define daddr_t as a 32-bit type, which is sad and stymies
cross-building disk images. Cast daddr_t to off_t before doing
arithmetic that overflows.

Reviewed by: arichardson, jrtc27, imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D27458