[SCSI] iscsi_boot_sysfs: have this module check for null on destruction

This moves the check for NULL boot_sets to the iscsi_boot_sysfs
module instead of having the drivers do it.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>