mm: warn about VmData over RLIMIT_DATA

This patch provides a way of working around a slight regression
introduced by commit 84638335900f ("mm: rework virtual memory
accounting").

Before that commit RLIMIT_DATA have control only over size of the brk
region.  But that change have caused problems with all existing versions
of valgrind, because it set RLIMIT_DATA to zero.

This patch fixes rlimit check (limit actually in bytes, not pages) and
by default turns it into warning which prints at first VmData misuse:

  "mmap: top (795): VmData 516096 exceed data ulimit 512000.  Will be forbidden soon."

Behavior is controlled by boot param ignore_rlimit_data=y/n and by sysfs
/sys/module/kernel/parameters/ignore_rlimit_data.  For now it set to "y".

[[email protected]: tweak kernel-parameters.txt text[
Signed-off-by: Konstantin Khlebnikov <[email protected]>
Link: http://lkml.kernel.org/r/20151228211015.GL2194@uranus
Reported-by: Christian Borntraeger <[email protected]>
Cc: Cyrill Gorcunov <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Vegard Nossum <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Quentin Casasnovas <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Willy Tarreau <[email protected]>
Cc: Pavel Emelyanov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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