libgfapi: In glfs_*chown, if uid or gid is -1, do not set the corresponding flag.

Change-Id: I23d0caa36c37dd5372f6e1d6a27c7904f4df547b
BUG: 1111563
Signed-off-by: Poornima Gurusiddaiah <pgurusid@redhat.com>
Reviewed-on: http://review.gluster.org/8115
Reviewed-by: Raghavendra Talur <rtalur@redhat.com>
Reviewed-by: Prashanth Pai <ppai@redhat.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Tested-by: Justin Clift <justin@gluster.org>
Reviewed-by: Shyamsundar Ranganathan <srangana@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>