src_plen for IPv4 routes imported from kernel has not been adjusted
to v4mapped encoding. Therefore IPv4 xroutes used an inconsistent encoding
which lead to failed comparisons when sending updates.

Routes received from neighbors with the same prefix as xroutes therefore
have been announced to neighbours instead.

This issue is fixed by converting src_plen on import.
Signed-off-by: Fabian Bläse <fabian@blaese.de>