dm mpath: don't return -EIO from dm_report_EIO

Instead just turn the macro into a helper for the warning message.
This removes an unnecessary assignment and will allow the next commit to
fix a place where -EIO is the wrong return value.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>