fix incorrect syscall usage in tester/mapping1 test

[BohdanQQ]

In the /app/tester's mapping1 test case, NULL is passed into the as_get_physical_mapping syscall.
This results in the syscall correctly returning EPERM, as down the call stack the return value is determined by

// uspace_dst is our incoming NULL
rc = !memcpy_to_uspace(uspace_dst, src, size) ? EPERM : EOK;

(as memcpy_to_uspace returns 0 on failure) and the (unexpected) failure of the test case.

As this use of the syscall (passing NULL as the only out argument) is not reflective of real usage,
I only adjusted the call site. I also fixed a comment in the assembly which seems incorrect according
to System V x86-64 ABI.