afsio: Remove redundant assignment

tbuf is assigned to during the initialisation phase of all of the
for loops that follow, so just remove this assignment

Caught by clang-analyzer

Change-Id: I3f2ffc8cee93768bd03e0abf3b391e1f6c45c70f
Reviewed-on: http://gerrit.openafs.org/9189
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>