NFSv4.2: Fix initialisation of struct nfs4_label

[ Upstream commit c528f70f504434eaff993a5ddd52203a2010d51f ]

The call to nfs4_label_init_security() should return a fully initialised
label.

Fixes: aa9c2669 ("NFS: Client implementation of Labeled-NFS")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>