target: bounds check XCOPY total descriptor list length

target: bounds check XCOPY total descriptor list length

spc4r37 6.4.3.5 states:
  If the combined length of the CSCD descriptors and segment descriptors
  exceeds the allowed value, then the copy manager shall terminate the
  command with CHECK CONDITION status, with the sense key set to ILLEGAL
  REQUEST, and the additional sense code set to PARAMETER LIST LENGTH
  ERROR.

This functionality can be tested using the libiscsi
ExtendedCopy.DescrLimits 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>