DARWIN: Fix setpag syscall error detection

DARWIN: Fix setpag syscall error detection

Darwin uses the same variable as the return value for
ioctl_afs_syscall, and as the last 'out' parmater, so the return value
for ioctl_afs_syscall will always overwrite whatever is written to the
last 'out' parameter. Instead, record two different variables, and
allow the returned error to overridte the error set as the last
parameter.

Reviewed-on: http://gerrit.openafs.org/3497
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 9da7f3cc73bf990427a9dd80ba2bbbb618383ea3)

Change-Id: I440d33e5cdd94dc5c7d12ddef1e63a05e53d787d
Reviewed-on: http://gerrit.openafs.org/3508
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>