scsi: Use struct scsi_lun in fc/fcp.h

This allows us to use scsilun_to_int without an ugly cast.

Fix up places that use scsilun_to_int on fcp->fc_lun accordingly.

In fc target, this leaves ft_cmd.lun unused, so remove it.

Signed-off-by: Andy Grover <agrover@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Kiran Patil <kiran.patil@intel.com>
Cc: James Bottomley <JBottomley@Parallels.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>