nfsd4: typo logical vs bitwise negate

This should be a bitwise negate here.  It silences a Sparse warning:
fs/nfsd/nfs4xdr.c:693:16: warning: dubious: x & !y

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>