cluster/afr: Preserve errno in case of failures on all subvols

        Backport of http://review.gluster.org/8984

Problem:
When quorum is enabled and the fop fails on all the subvolumes,
op_errno is set to EROFS which overrides the actual errno returned
from bricks.

Fix:
Don't override the errno when fop fails on all subvols.

Change-Id: I61e57bbf1a69407230ec172a983de18d1c624fd2
BUG: 1162122
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/9087
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>