mm, oom: add comment for why oom_adj exists

/proc/pid/oom_adj exists solely to avoid breaking existing userspace
binaries that write to the tunable.

Add a comment in the only possible location within the kernel tree to
describe the situation and motivation for keeping it around.

Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>