FROMLIST: arm: mm: support ARCH_MMAP_RND_BITS.

(cherry picked from commit https://lkml.org/lkml/2015/12/21/341

)

arm: arch_mmap_rnd() uses a hard-code value of 8 to generate the
random offset for the mmap base address.  This value represents a
compromise between increased ASLR effectiveness and avoiding
address-space fragmentation. Replace it with a Kconfig option, which
is sensibly bounded, so that platform developers may choose where to
place this  compromise. Keep 8 as the minimum acceptable value.

Bug: 24047224
Signed-off-by: Daniel Cashman <dcashman@android.com>
Signed-off-by: Daniel Cashman <dcashman@google.com>
Git-commit: 983bb4778143ef60deaeaffba320d316b8b3d263
Git-repo: https://android.googlesource.com/kernel/common.git


[schikk@codeaurora.org conflict resolved]
CRs-Fixed: 1066871
Change-Id: If5ae03b7db3742ff7ec607ae5e9b44c7a540e79c
Signed-off-by: Swetha Chikkaboraiah <schikk@codeaurora.org>