snapview-server: get the handle if its absent before doing any fop

* Now that NFS server does inode linking in readdirp, it can resolve the
  gfid (i.e. find the right inode from its inode table) present in the
  filehandle sent by the NFS client on which a fop came. So instead of
  sending the lookup on that entry, it directly sends the fop. But
  snapview-server does not get the handle for the entries in readdirp
  (because doing a lookup on each entry via gfapi would be costly. So it
   waits till a lookup is done on that inode, to get the handle and the
   fs instance and fill it in the inode context). So when NFS resoves the
   gfid and directly sends the fop, snapview-server will not be able to
   perform the fop as the inode contet would not contain the fs instance
   and the handle. So fops should check for the handle before doing gfapi
   calls. If the handle and fs instance are not present in the inode context
   they should get them by doing an explicit lookup on the entry.

rebase of the patch http://review.gluster.org/#/c/8324/

Change-Id: I70c9c8edb2e7ddad79cf6ade3e041b9d02241cd1
BUG: 1143961
Reviewed-on: http://review.gluster.org/8768
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>