Prevent the topdown allocator from allocating mmap areas all the way
down to address zero.
We still allow a MAP_FIXED mapping of page 0 (needed for various things,
ranging from Wine and DOSEMU to people who want to allow speculative
loads off a NULL pointer).
Tested by Chris Wright.
Signed-off-by: Linus Torvalds <torvalds@osdl.org>