nvme completion handling refactor and fix v3

[Christoph Hellwig]

Hi all,

the first two patches refactor the common nvme completion code
to be a little less obsfucated, and the third one is the fix
from Chao to redirect to a different path ported on top of these
changes, which now make the fix very simple.

Changes since v2:
 - fix a comment typo
 - drop incorrect comment masking in nvme_decide_disposition
 - add a comment to nvme_is_path_error

Changes since v1:
 - add a new patch to handle the path related errors using the
   SCT class