ipsec: Validate the protocol identifier in ipsec4_ctlinput()

[markj]

Summary

key_allocsa() expects to handle only IPSec protocols and has an
assertion to this effect. However, ipsec4_ctlinput() has to handle
messages from ICMP unreachable packets and was not validating the
protocol number. In practice I believe such a packet would simply fail
to match any SADB entries and would thus be ignored.

Reported by: [e-mail]