riscv: Make sure an exception is raised if a pte is malformed

riscv: Make sure an exception is raised if a pte is malformed

As per the specification, in 64-bit, if any of the pte reserved bits
60-54 is set an exception should be triggered (see 4.4.1, "Addressing and
Memory Protection"). In addition, we must check the napot/pbmt bits are
not set if those extensions are not active.

Reported-by: Andrea Parri <andrea@rivosinc.com>
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230420150220.60919-1-alexghiti@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>