ipv4: save cpu cycles from check_leaf()

Compiler is not smart enough to avoid double BSWAP instructions in
ntohl(inet_make_mask(plen)).

Lets cache this value in struct leaf_info, (fill a hole on 64bit arches)

With route cache disabled, this saves ~2% of cpu in udpflood bench on
x86_64 machine.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>