NFSv4.2: Clear FATTR4_WORD2_SECURITY_LABEL when done decoding

We need to clear the FATTR4_WORD2_SECURITY_LABEL bitmap flag
irrespective of whether or not the label is too long.

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