libceph: eliminate sparse warnings

Eliminate most of the problems in the libceph code that cause sparse
to issue warnings.
    - Convert functions that are never referenced externally to have
      static scope.
    - Pass NULL rather than 0 for a pointer argument in one spot in
      ceph_monc_delete_snapid()

This partially resolves:
    http://tracker.ceph.com/issues/4184

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
diff --git a/net/ceph/messenger.c b/net/ceph/messenger.c
index 771d4c9..ed9e237 100644
--- a/net/ceph/messenger.c
+++ b/net/ceph/messenger.c
@@ -223,7 +223,7 @@
  */
 static struct workqueue_struct *ceph_msgr_wq;
 
-void _ceph_msgr_exit(void)
+static void _ceph_msgr_exit(void)
 {
 	if (ceph_msgr_wq) {
 		destroy_workqueue(ceph_msgr_wq);