tsan: fix another latent race size bug in test

The test contains a race in memset.
The size of reported race depends on how the accessed
memory range split into granules inside of tsan runtime.
The test used to report access of size 8, because presumably
the buffer ended up being aligned to 8 bytes. But after
some unrelated changes this test started to report accesses
of size 1 (presumably .data layout changed), which makes
the test fail.
Guarantee alignment of the buf object explicitly.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D107149