bpf: Fix IPv6 dport byte-order in bpf_sk_lookup

Commit 6acc9b43 ("bpf: Add helper to retrieve socket in BPF")
mistakenly passed the destination port in network byte-order to the IPv6
TCP/UDP socket lookup functions, which meant that BPF writers would need
to either manually swap the byte-order of this field or otherwise IPv6
sockets could not be located via this helper.

Fix the issue by swapping the byte-order appropriately in the helper.
This also makes the API more consistent with the IPv4 version.

Fixes: 6acc9b43 ("bpf: Add helper to retrieve socket in BPF")
Signed-off-by: Joe Stringer <joe@wand.net.nz>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>