fio: fix some struct alignment issues

fio: fix some struct alignment issues

Fix unaligned/misaligned accesses related to struct thread_stat and
struct jobs_eta seen when running a build produced by
CC=~/clang-3.9/build/bin/clang ./configure --disable-optimizations \
   --extra-cflags="-D__compiler_offsetof=__builtin_offsetof \
   -fsanitize=undefined"

and add to the compile time asserts to make these problems more visible.

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>