Improve error handling around duplicate lun and port enable.

This fixes kernel panic if port enabled twice and then disabled.

MFC after:	1 week

(cherry picked from commit 4bfbbd334feed6b91c968bc91763eef91de8ee42)

Conflicts:
	sys/cam/ctl/scsi_ctl.c
Fix residual copy/paste in r274080.

MFC after:	1 week

(cherry picked from commit 57bc647ef2a75a47c8e3caedb3ef05307600b78f)

Conflicts:
	sys/cam/ctl/scsi_ctl.c