RFC: mbuf: Add m_len assertion to mtod() and mtodo()

Hi FreeBSD developers,

After the recent findings that a network module may end up doing things 
like mtod(m, struct ip *) over an empty mbuf in a chain, an idea has 
come to add m_len assertion to the existing mtod() and mtodo() macros. 
Thus, mtod() would panic if m->m_len < sizeof(struct ip) in the example.

The current implementation proposal is here:

	https://reviews.freebsd.org/D46684


The high level technical plan for this long path is as follows:
1. Fix compilation cases
2. Fix runtime cases, e.g. mtod() can be used before m_len is prepared
3. Land the assertion


The very first inconvenience found is that it will make mtod() 
unavailable for the following two use cases:

- void pointer
	mtod(m, void *)

- work with m_data pointer itself:
	mtod(m, vm_offset_t)
	mtod(m, uintptr_t) & 3

Currently, 116 void* cases and 60 m_data pointer cases are found [1]. 
And they are targeted to be re-worked. It's planned to consider each 
case because of something could be not just a literal macro expansion, 
e.g. mtod() & 3 examples could be changed to something like 
m_alignment(m) & 3 or m_is_aligned(m, 3).


It would be appreciated to receive comments, opinions, and suggestions 
before starting work on the respective changes.


[1] The cases found: https://github.com/ihoro/freebsd-src/pull/31/files


Best regards,
igoro