rpc_pipefs: fix double-dput()

if we ever hit rpc_gssd_dummy_depopulate() dentry passed to
it has refcount equal to 1.  __rpc_rmpipe() drops it and
dput() done after that hits an already freed dentry.

Cc: stable@kernel.org
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

net/sunrpc/rpc_pipe.c

@@ -1375,6 +1375,7 @@ rpc_gssd_dummy_depopulate(struct dentry *pipe_dentry)
 	struct dentry *clnt_dir = pipe_dentry->d_parent;
 	struct dentry *gssd_dir = clnt_dir->d_parent;
 
+	dget(pipe_dentry);
 	__rpc_rmpipe(d_inode(clnt_dir), pipe_dentry);
 	__rpc_depopulate(clnt_dir, gssd_dummy_info_file, 0, 1);
 	__rpc_depopulate(gssd_dir, gssd_dummy_clnt_dir, 0, 1);