[SCSI] iscsi_boot_sysfs: Fix a memory leak in iscsi_boot_destroy_kset()

Load and unload iscsi_ibft module will cause kernel memory leak, fix it
in scsi/iscsi_boot_sysfs.c iscsi_boot_destroy_kset().

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