If the shm region is not mapped at shmaddr, EINVAL.
Do not unmap the region until the syscall succeeds.
Use mmap_reserve_or_unmap to preserve reserved_va semantics.

Tested-by: Helge Deller <deller@gmx.de>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>