Modify the binder to request 1M - 2 pages instead of 1M. The backing store

in the kernel requires a guard page, so 1M allocations fragment memory very
badly.  Subtracting a couple of pages so that they fit in a power of
two allows the kernel to make more efficient use of its virtual address space.

Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>