dhcpv4: make the cookie explicit in struct dhcpv4_message

dhcpv4: make the cookie explicit in struct dhcpv4_message

This makes the struct more explicit while allowing a number of magic offsets to
be removed.

Note that adding the packed attribute to struct dhcpv4_message is necessary
because the incoming data is cast directly to the struct. Adding the attribute
will give the struct an alignment of 1 instead of 4 (the normal alignment of
struct in_addr and uint32_t).

In the beginning of dhcpv4_handle_msg(), we cast "void *data" to struct
dhcpv4_message, but we can't know the alignment of "*data" (meaning it has
alignment 1). And by casting "*data" to a struct, we're actually promising the
compiler that "*data" has the same alignment as the struct.

Later, the address of members of struct dhcpv4_message are passed to other
functions, but since struct dhcpv4_message can be at any memory location, it's
not a given that the pointers are aligned. This issue only becomes visible once
the packed attribute is added, which is why the temporary (aligned) struct
in_addr variables are added in the WITH_UBUS blocks.

Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/266
Signed-off-by: Álvaro Fernández Rojas <[email protected]>