iscsi: Use struct scsi_lun in iscsi structs instead of u8[8]

struct scsi_lun is also just a struct with an array of 8 octets (64 bits)
but using it instead in iscsi structs lets us call scsilun_to_int
without a cast, and also lets us copy it using assignment, instead of
memcpy().

Signed-off-by: Andy Grover <agrover@redhat.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>