selftests: riscv: Generalize mm selftests

The behavior of mmap on riscv is defined to not provide an address that
uses more bits than the hint address, if provided. Make the tests
reflect that.

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Link: https://lore.kernel.org/r/20240130-use_mmap_hint_address-v3-2-8a655cfa8bcb@rivosinc.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>