Unify `nvme_*_passthru*` functions

[MaisenbacherD]

We should hide the nvme_*_passthru64 ioctl functions from the user. The library should handle the usage of the nvme_*_passthru vs. nvme_*_passthru64 internally and should expose a unified API to the user.
Use 64 bit variant by default.

As discussed with the community at ALPSS

[ikegami-t]

By the way I do not understand about the difference between nvme_*_passthru and nvme_*_passthru64 enough so please let me confirm below.

1. The structures themself diference is just the result variable size.

struct nvme_passthru_cmd {
	__u8	opcode;
...
	__u32	timeout_ms;
	__u32	result;
};

struct nvme_passthru_cmd64 {
	__u8    opcode;
...
	__u32   timeout_ms;
	__u32   rsvd2;
	__u64   result;
};

2. Admin Command and I/O Command Common CQE described as below.
   
3. Fabrics Command Common CQE desribed as below.
   
4. My understanding is the result set (Admin Command and I/O Command Common) CQE DW3 (32-bit) value. For the Fabrics Command Common CQE the result needed to set 64-bit value for example FRTS?

(Added)
5. Just reconfirmed as Zone Append command CQE described as below.

7. So now I am thinking as nvme_passthru_cmd64 set the result as Dword 0/1 ALBA (64-bit). (So my understanding was not correct for the CQE DW3 as now I am thinking the nvme_passthru_cmd sets result to CQE DW0 but not DW3.

If you have any comment or advice please let me know. Thank you.

(Readded)
6. Confirmed the nvme_completion definition as below then understood as the result set CQ DW0 (16-bit or 32-bit) or CQ DW0/1 (64-bit).

struct nvme_completion {
	/*
	 * Used by Admin and Fabrics commands to return data:
	 */
	union nvme_result {
		__le16	u16;
		__le32	u32;
		__le64	u64;
	} result;
	__le16	sq_head;	/* how much of this queue may be reclaimed */
	__le16	sq_id;		/* submission queue that generated this entry */
	__u16	command_id;	/* of the command which completed */
	__le16	status;		/* did the command fail, and if so, why? */
};

[igaw]

Exactly, the idea is just to use the largest type for result to everywhere. The u64 variant was added with fabrics, now we are just going to use it.

[ikegami-t]

I see. If needed we can also use the union variable as same with nvme_completion I think.