[NETFILTER]: x_tables: fix xt_register_table error propagation

When xt_register_table fails the error is not properly propagated back.
Based on patch by Lepton Wu <ytht.net@gmail.com>.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>