rpc_pipe: clean failure exits in fill_super

rpc_pipe: clean failure exits in fill_super

->kill_sb() will be called immediately after a failure return
anyway, so we don't need to bother with notifier chain and
rpc_gssd_dummy_depopulate().  What's more, rpc_gssd_dummy_populate()
failure exits do not need to bother with __rpc_depopulate() -
anything added to the tree will be taken out by ->kill_sb().

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>