accel/tcg: Take mmap lock in the whole cpu_memory_rw_debug() function

accel/tcg: Take mmap lock in the whole cpu_memory_rw_debug() function

Simplify user implementation of cpu_memory_rw_debug() by
taking the mmap lock globally. See commit 87ab2704296
("linux-user: Allow gdbstub to ignore page protection")
for why this lock is necessary.

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20250217130610.18313-4-philmd@linaro.org>