net: Fix warnings caused by MAX_SKB_FRAGS change.

After commit a715dea3 ("net: Always
allocate at least 16 skb frags regardless of page size"), the value
of MAX_SKB_FRAGS can now take on either an "unsigned long" or an
"int" value.

This causes warnings like:

net/packet/af_packet.c: In function ‘tpacket_fill_skb’:
net/packet/af_packet.c:948: warning: format ‘%lu’ expects type ‘long unsigned int’, but argument 2 has type ‘int’

Fix by forcing the constant to be unsigned long, otherwise we have
a situation where the type of a system wide constant is variable.

Signed-off-by: David S. Miller <davem@davemloft.net>