mm, oom: reintroduce /proc/pid/oom_adj

This is mostly a revert of 01dc52eb ("oom: remove deprecated oom_adj")
from Davidlohr Bueso.

It reintroduces /proc/pid/oom_adj for backwards compatibility with earlier
kernels.  It simply scales the value linearly when /proc/pid/oom_score_adj
is written.

The major difference is that its scheduled removal is no longer included
in Documentation/feature-removal-schedule.txt.  We do warn users with a
single printk, though, to suggest the more powerful and supported
/proc/pid/oom_score_adj interface.

Reported-by: Artem S. Tashkinov <t.artem@lycos.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>