nfsd: ensure that nfsd4_fattr_args.context is zeroed out

If nfsd4_encode_fattr4 ends up doing a "goto out" before we get to
checking for the security label, then args.context will be set to
uninitialized junk on the stack, which we'll then try to free.
Initialize it early.

Fixes: f59388a5 ("NFSD: Add nfsd4_encode_fattr4_sec_label()")
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>