Shrink struct task_struct::oomkilladj

oomkilladj is int, but values which can be assigned to it are -17, [-16,
15], thus fitting into s8.

While patch itself doesn't help in making task_struct smaller, because of
natural alignment of ->link_count, it will make picture clearer wrt futher
task_struct reduction patches.  My plan is to move ->fpu_counter and
->oomkilladj after ->ioprio filling hole on i386 and x86_64.  But that's
for later, because bloated distro configs need looking at as well.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>