[SCSI] scsi_debug: fix resp_xdwriteread() return value when running out of memory

When resp_xdwriteread() can't allocate temporary buffer, it returns -1.
But the return value is used as scsi status code and -1 is not
interpreted as correct code.

target_core_mod has similar xdwriteread emulation code. So this mimics
what target_core_mod does for xdwriteread when running out of memory.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Reviewed-by: Douglas Gilbert <dgilbert@interlog.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>