iproute: fix parsing and matching of of "short" IP addrs like 10/8

iproute: fix parsing and matching of of "short" IP addrs like 10/8

function                                             old     new   delta
print_route                                         1613    1813    +200
get_addr_1                                           209     258     +49
get_prefix                                           393     356     -37
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(add/remove: 0/0 grow/shrink: 2/1 up/down: 249/-37)           Total: 212 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>