dht: dht_lookup_dir_cbk should set op_errno as local->op_errno

Two glusterfs clients return inconsistent errnos when the bricks of the volume
were down. Consider two gluster mounts. Mount 1 was done when the bricks were
online. Mount 2 was done after the bricks were killed, (using the 'glusterfs'
command instead of the mount script).

For any request, mount 1 will return ENOTCONN, where as mount 2 will return
ENOENT.

This happens because for the 2nd mount, a fuse would send a lookup on '/' for
any request, as it hadn't been done yet. The client xlator returns ENOTCONN,
but the dht_lookup_dir_cbk changed this to ENOENT unconditionally when
aggregating. So, fuse returned ENOENT, even though the errno should have been
ENOTCONN.

Change-Id: I4b7a6d84ce5153045a807fccc01485afe0377117
BUG: 1019095
Signed-off-by: Kaushal M <kaushal@redhat.com>
Reviewed-on: http://review.gluster.org/6072
Reviewed-by: Anand Avati <avati@redhat.com>
Tested-by: Anand Avati <avati@redhat.com>