cluster/distribute: If cached_subvol is down, return ENOTCONN in lookup

When we follow a linkfile, and the lookup returns a ENOTCONN error, return
the error, as the cached subvol is down, and lookup_everywhere wont succeed,
but actually ends up clearing the linkfile, and clearing the namespace.

Change-Id: I772bf71531bc646e8fb62d3e8549a5fe0f3896da
BUG: 893378
Signed-off-by: shishir gowda <sgowda@redhat.com>
Reviewed-on: http://review.gluster.org/4383
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>