BACKPORT: FROMLIST: arm64: mm: support ARCH_MMAP_RND_BITS.

(cherry picked from commit https://lkml.org/lkml/2015/12/21/340

)

arm64: arch_mmap_rnd() uses STACK_RND_MASK 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 default values as new minimums.

Bug: 24047224
Signed-off-by: Daniel Cashman <dcashman@android.com>
Signed-off-by: Daniel Cashman <dcashman@google.com>
Git-commit: 0896a27b57016186a34b634198d52fe61e007355
Git-repo: https://android.googlesource.com/kernel/common.git


[schikk@codeaurora.org iconflict resolved]
CRs-Fixed: 1066871
Change-Id: Id49b6a5cc1a71ace4bde321f487237e68ff81c24
Signed-off-by: Swetha Chikkaboraiah <schikk@codeaurora.org>