[sanitizer] Correct name length computation for some Fuchsia vmos

Summary:
This was missed during the review of D38595, but the vmo name size computation
should use internal_strlen, not sizeof, otherwise we end up with 7 character
names.

Reviewers: mcgrathr, flowerhack

Reviewed By: mcgrathr

Subscribers: kubamracek, delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D45525