NFSv4.2: Always decode the security label

If the server returns a reply that includes a security label, then we
must decode it whether or not we can store the results.

Fixes: 1e2f67da ("NFS: Remove the nfs4_label argument from decode_getattr_*() functions")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>