darwin afscall syscall should set retval

darwin afscall syscall should set retval

because of how the ioctl based syscall works, passing back a real
return value should be done this way, as it is for the other other
afs syscall subcalls. matters for userspace handlers.

Change-Id: I894b4bd633ce4823b1ce906861251fcbb3949af2
Reviewed-on: http://gerrit.openafs.org/1381
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>