s390/mmap: randomize mmap base for bottom up direction

s390/mmap: randomize mmap base for bottom up direction

Implement mmap base randomization for the bottom up direction, so ASLR
works for both mmap layouts on s390.  See also df54d6fa54 ("x86
get_unmapped_area(): use proper mmap base for bottom-up direction").

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Radu Caragea <sinaelgl@gmail.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Chris Metcalf <cmetcalf@tilera.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>