target: support XCOPY requests without parameters

SPC4r37 6.4.1 EXTENDED COPY(LID4) states (also applying to LID1 reqs):
  A parameter list length of zero specifies that the copy manager shall
  not transfer any data or alter any internal state, and this shall not
  be considered an error.

This behaviour can be tested using the libiscsi ExtendedCopy.ParamHdr
test.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>