[NETFILTER]: x_tables: switch xt_target->checkentry to bool

Switch the return type of target checkentry functions to boolean.

Signed-off-by: Jan Engelhardt <jengelh@gmx.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>