libglusterfs: Maintain open-fd-count in inode.

Change-Id: I643d02959f92e40f68a53baf165753ed20f8b3e0
BUG: 908146
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/4468
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>