[NETFILTER]: x_tables: change xt_table_register() return value convention

Switch from 0/-E to ptr/PTR_ERR convention.

Signed-off-by: Alexey Dobriyan <adobriyan@sw.ru>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>