libgfapi: succeed lookup of "/.."

For the root dir, ".." should resolve to itself.

i.e. when
        glfs_h_lookupat (fs, NULL, "/..", &stat)
(or)
        glfs_h_lookupat (fs, root, "..", &stat)

is performed, stat must contain root dir's information.

Change-Id: I1c92091cdc4ff00e6b17e5fa349009c6dfc441c1
BUG: 1114814
Signed-off-by: Ravishankar N <ravishankar@redhat.com>
Reviewed-on: http://review.gluster.org/8207
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Reviewed-by: Poornima G <pgurusid@redhat.com>
Reviewed-by: Prashanth Pai <ppai@redhat.com>
Reviewed-by: Raghavendra Talur <rtalur@redhat.com>