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>

riscv: kselftests: Fix mm build by removing testcases subdirectory

kselftests fails to build because the mm/testcases subdirectory is not
created and then the compiler fails to output the binary there.

So fix this by simply removing this subdirectory which is not very
useful.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Link: https://lore.kernel.org/r/20230915100113.13131-1-alexghiti@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>