selftests/sgx: Fix Q1 and Q2 calculation in sigstruct.c

Q1 and Q2 are numbers with *maximum* length of 384 bytes. If the
calculated length of Q1 and Q2 is less than 384 bytes, things will
go wrong.

E.g. if Q2 is 383 bytes, then

1. The bytes of q2 are copied to sigstruct->q2 in calc_q1q2().
2. The entire sigstruct->q2 is reversed, which results it being
   256 * Q2, given that the last byte of sigstruct->q2 is added
   to before the bytes given by calc_q1q2().

Either change in key or measurement can trigger the bug. E.g. an
unmeasured heap could cause a devastating change in Q1 or Q2.

Reverse exactly the bytes of Q1 and Q2 in calc_q1q2() before returning
to the caller.

Fixes: 2adcba79

 ("selftests/x86: Add a selftest for SGX")
Link: https://lore.kernel.org/linux-sgx/20210301051836.30738-1-tianjia.zhang@linux.alibaba.com/
Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>