mips: use generic mmap top-down layout and brk randomization

mips: use generic mmap top-down layout and brk randomization

mips uses a top-down layout by default that exactly fits the generic
functions, so get rid of arch specific code and use the generic version by
selecting ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT.

As ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT selects ARCH_HAS_ELF_RANDOMIZE,
use the generic version of arch_randomize_brk since it also fits.  Note
that this commit also removes the possibility for mips to have elf
randomization and no MMU: without MMU, the security added by randomization
is worth nothing.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Alexandre Ghiti <[email protected]>
Acked-by: Paul Burton <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Reviewed-by: Luis Chamberlain <[email protected]>
Cc: Albert Ou <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: James Hogan <[email protected]>
Cc: Palmer Dabbelt <[email protected]>
Cc: Ralf Baechle <[email protected]>
Cc: Russell King <[email protected]>
Cc: Will Deacon <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>