syscall: fix arithmetic errors in assembly for seek function for 64-bit Plan 9

Offsets for return values from seek were miscalculated
and a translation from 32-bit code for error handling
was incorrect.

R=rsc, rminnich, npe
CC=golang-dev
https://codereview.appspot.com/7181045

Committer: Russ Cox <rsc@golang.org>