libdtrace: Fix TCP data offset handling in the tcpinfo_t translator

The header gives an offset in 32-bit words, and the translator is
supposed to convert that to a byte count. But, the conversion was
incorrect.

Reviewed by: tuexen, rscheff
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43264

(cherry picked from commit c3268c23de45b75b44102a02e96f44b0ff2f6ba7)