DARWIN: Use tb->code_raw for BOP_MOVE

DARWIN: Use tb->code_raw for BOP_MOVE

Currently, BOP_MOVE communicates its error code to the requestor via
the 'retval' field in struct afs_uspc_param, and we assume ptr_parm[0]
of the given brequest is for a struct afs_uspc_param. But this is
unnecessary, since struct brequest already has fields for error codes;
namely, code_raw and code_checkcode.

To avoid afs_BackgroundDaemon needing to interpret ptr_parm[0] in this
way (and assuming the type of the pointer's target), change BOP_MOVE
to just use the code_raw field for error codes, instead of
interpreting ptr_parm[0]. This makes it easier to add more AFS_NEW_BKG
background operations that do not pass a struct afs_uspc_param in the
brequest parameters.

Change-Id: I90a564468862142777159fbb78234744840b59fb
Reviewed-on: https://gerrit.openafs.org/13280
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>