acpi/apei/erst: Add missing iounmap() on error in erst_exec_move_data()

Add the missing iounmap() before return from erst_exec_move_data()
in the error handling case.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>