fuse: protect checking/modifying graph->used with a mutex

While reviewing http://review.gluster.org/#/c/9060/ we noticed that fuse
checks/modifies graph->used in a unlocked way. There is a potential race
in case when fuse_graph_setup() gets called from two different threads
(currently  unlikely, but poller/notify() might change in future?).

In libgfapi we came to the conclusion that each glfs structure can be
used to protect the checking/updating of graph->used. In fuse this would
be the fuse_private_t with sync_mutex.

BUG: 1170643
Change-Id: If5ab5468d22fdb92cfb24a469f538f63f12baf78
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/9237
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Raghavendra G <rgowdapp@redhat.com>