target: Make target_do_xcopy failures return INVALID_PARAMETER_LIST

This patch changes target_do_xcopy() to properly return
TCM_INVALID_PARAMETER_LIST instead of TCM_INVALID_CDB_FIELD
for failures related to the EXTENDED_COPY parameter list parsing.

Also, move struct xcopy_op allocation ahead of kmapping to
handle the special TCM_OUT_OF_RESOURCES case.

Reported-by: Thomas Glanzmann <thomas@glanzmann.de>
Reported-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>