cluster/afr: Fail readv on data-split-brain

Problem:
Afr prevents opens on a file in split-brian but the
fd that is already open still has the capability to perform
both reads and writes to the file.

Fix:
Fail readvs on a file with EIO.

Change-Id: I8e07f24c36fab800499b36ab374f984b743332cd
BUG: 873962
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/4199
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>