This function can fail, which makes it more like ftrylockfile
or pthread_mutex_trylock than flockfile or pthread_mutex_lock,
so rename it.

To closer match the other trylock functions, release rcu_read_lock
along the failure path, so that qemu_log_unlock need not be called
on failure.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220417183019.755276-8-richard.henderson@linaro.org>