tools: fix tabletool reset of nonexistent sessionmap

If the object didn't exist, the omap clear was failing
and preventing the subsueent omap set header from
executing.  Set the FAILOK flag on the omap clear
sub-operation.

Signed-off-by: John Spray <john.spray@redhat.com>

src/tools/cephfs/TableTool.cc

@@ -353,6 +353,7 @@ public:
     // Compose a transaction to clear and write header
     librados::ObjectWriteOperation op;
     op.omap_clear();
+    op.set_op_flags(librados::OP_FAILOK);
     op.omap_set_header(header_bl);
     
     return io->operate(object_name, &op);