[NETFILTER]: ctnetlink: Fix expectaction mask dumping

The expectation mask has some particularities that requires a different
handling. The protocol number fields can be set to non-valid protocols,
ie. l3num is set to 0xFFFF. Since that protocol does not exist, the mask
tuple will not be dumped. Moreover, this results in a kernel panic when
nf_conntrack accesses the array of protocol handlers, that is PF_MAX (0x1F)
long.

This patch introduces the function ctnetlink_exp_dump_mask, that correctly
dumps the expectation mask. Such function uses the l3num value from the
expectation tuple that is a valid layer 3 protocol number. The value of the
l3num mask isn't dumped since it is meaningless from the userspace side.

Thanks to Yasuyuki Kozakai and Patrick McHardy for the feedback.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>