FROMLIST: mm: mmap: Add new /proc tunable for mmap_base ASLR.

(cherry picked from commit https://lkml.org/lkml/2015/12/21/337

)

ASLR  only uses as few as 8 bits to generate the random offset for the
mmap base address on 32 bit architectures. This value was chosen to
prevent a poorly chosen value from dividing the address space in such
a way  as to prevent large allocations. This may not be an issue on all
platforms. Allow the specification of a minimum number of bits so that
platforms desiring greater ASLR protection may determine where to place
the trade-off.

Bug: 24047224
Signed-off-by: Daniel Cashman <dcashman@android.com>
Signed-off-by: Daniel Cashman <dcashman@google.com>
Git-commit: 00ead9ddada26be1726539b1ead14abf974d235d
Git-repo: https://android.googlesource.com/kernel/common.git


[schikk@codeaurora.org conflict resolved]
CRs-Fixed: 1066871
Change-Id: Iecbfb9abfe8f3f27d83a85989145d7fee05806a0
Signed-off-by: Swetha Chikkaboraiah <schikk@codeaurora.org>