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/mon_client.c b/net/ceph/mon_client.c
index 812eb3b..aef5b10 100644
--- a/net/ceph/mon_client.c
+++ b/net/ceph/mon_client.c
@@ -697,7 +697,7 @@
 			    u32 pool, u64 snapid)
 {
 	return do_poolop(monc,  POOL_OP_CREATE_UNMANAGED_SNAP,
-				   pool, snapid, 0, 0);
+				   pool, snapid, NULL, 0);
 
 }