accel/tcg: Fix sense of read-only probes in ldst_atomicity

accel/tcg: Fix sense of read-only probes in ldst_atomicity

In the initial commit, cdfac37be0d, the sense of the test is incorrect,
as the -1/0 return was confusing.  In bef6f008b981, we mechanically
invert all callers while changing to false/true return, preserving the
incorrectness of the test.

Now that the return sense is sane, it's easy to see that if !write,
then the page is not modifiable (i.e. most likely read-only, with
PROT_NONE handled via SIGSEGV).

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>