cluster/dht: Handle linkfile creation with EEXIST error

If linkfile create fails with EEXISTS, then check if the file
is a linkfile for the same file. If not, return the error

Change-Id: Iab42db54422dea69de0049b5196365e65edadd91
BUG: 966858
Signed-off-by: shishir gowda <sgowda@redhat.com>
Reviewed-on: http://review.gluster.org/5060
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>