NFSv4.2: Fix a memory stomp in decode_attr_security_label

We must not change the value of label->len if it is zero, since that
indicates we stored a label.

Fixes: b4487b93 ("nfs: Fix getxattr kernel panic and memory overflow")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>