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

ASLR currently only uses 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@google.com>
Change-Id: I1e173f2d890484421298722c5a045c65ca5af581