nvme: refactor PI tagging for uniform behavior

[MaisenbacherD]

Introduce init_pi_tags helper function that is called for all IO commands, to ensure uniform behavior.

Renaming related variables to match the spec.

[MaisenbacherD]

@igaw Not sure how to address the check patch warnings about the exceeding char limit. I tried to fit the changes to the surrounding codebase.
I could fix those warning up by also patching the lines that I didn't touch to have a uniform style. :)

[igaw]

Just ingore those checkpatch warnings. It's just a stupid reporting tool and we should not blindly follow it. Your changes look good to me. The only thing I would probably change is the overlong lines for the function definition.

[igaw]

Would you mind to rebase it to the current master. Due the passthru updates some changes are necessary, though it should be fairly simple :)

[MaisenbacherD]

Done :)
I made sure to shorten the function definitions.

The rebase even discovered a bug in init_pi_tags. It accepts the cmd as a pointer and I previously passed a reference of that pointer to the underlying nvme_init_var_size_tags function, which was not correct. The explicit casting to nvme_passthru_cmd64 * covered up this mistake.

[igaw]

Yeah, type casts have always a bit of a smell. Nice, thanks!