diff: Fix device case.
We already fell back to Stone for FIFOs, but we actually need to fall
back to Stone for everything except regular files, because libdiff's
atomizer needs to know the size of its input in advance, and neither
FIFOs nor devices can be trusted to report their size.
MFC after: 1 week
Reported by: mav
Reviewed by: mav, allanjude
Differential Revision: https://reviews.freebsd.org/D48181