target/xtensa: Make use of 'segment' in pptlb helper less confusing

target/xtensa: Make use of 'segment' in pptlb helper less confusing

Coverity gets confused about the use of the 'segment' variable in the
pptlb helper function: it thinks that we can take a code path where
we first initialize it:
  unsigned segment = XTENSA_MPU_PROBE_B;  // 0x40000000
and then use that value as a shift count:
  } else if (nhits == 1 && (env->sregs[MPUENB] & (1u << segment))) {

In fact this isn't possible, beacuse xtensa_mpu_lookup() is passed
'&segment', and it uses that as an output value, which it will always
set if it returns nonzero.  But the way the code is currently written
is confusing to a human reader as well as to Coverity.

Instead of initializing 'segment' at the top of the function with a
value that's only used in the "nhits == 0" code path, use the
constant value directly in that code path, and don't initialize
segment.  This matches the way we use xtensa_mpu_lookup() in its
other callsites in get_physical_addr_mpu().

Resolves: Coverity CID 1547589

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Max Filippov <jcmvbkbc@gmail.com>
Message-id: 20240723151454.1396826-1-peter.maydell@linaro.org