afr: conditionally prioritize EIO errors over ENOENT

The most important errno logic historically only prioritized ESTALE
over ENOENT. Commit c8c0942d added EIO prioritization over ENOENT
to ensure that split-brain was reported when it occurs in
conjunction with bricks missing the file entry. The unintended side
effect of this change is that (non split-brain) EIO errors reported
from the bricks themselves are now reported to the client when the
expectation is that afr should squash said errors in favor of
marking the file inconsistent.

The high-level problem is that EIO is overloaded with different
meanings from different contexts. This commit adds an eio parameter
to the errno priority logic to conditionally flag when EIO is of
higher priority and should be propagated to the client.

BUG: 892730
Change-Id: Ib692a8a1f1737ef190d57894f392ec53ffb33aab
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-on: http://review.gluster.org/4376
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>