mount/fuse: perform lookup() on inodes linked through readdirplus

Some xlators still require lookup() fop to be sent for proper working.
This patch remembers inodes which have been linked through readdiprlus
and makes the resolver send lookups on them.

Also, introduce and use context count for inode table.

Change-Id: Ibe8a04a659539d90dfc794521b51bf2bda017a0b
BUG: 979910
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5267
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-on: http://review.gluster.org/5806