In order to support ongoing work to implement variable-size mbufs, and

more generally make it easier to extend 'struct mbuf in the future', make
a number of changes to the data structure:

- As we anticipate embedding mbufs headers within variable-size regions of
  memory in the future, change the definitions of byte arrays embedded in
  mbufs to be of size [0] rather than [MLEN] and [MHLEN].  In fact, the
  cxgbe driver already uses 'struct mbuf' on the front of other storage
  sizes, but we would like the global mbuf allocator do be able to do this
  as well.

- Fold 'struct m_hdr' into 'struct mbuf' itself, eliminating a set of
  macros that aliased 'mh_foo' field names to 'm_foo' names such as
  'm_next'.  These present a particular problem as we would like to add
  new mbuf-header fields -- e.g., 'm_size' -- that, if similarly named via
  macros, would introduce collisions with many other variable names in the
  kernel.

- Rename 'struct m_ext' to 'struct struct_m_ext' so that we can add
  compile-time assertions without bumping into the still-extant 'm_ext'
  macro.

- Remove the MSIZE compile-time assertion for 'struct mbuf', but add new
  assertions for alignment of embedded data arrays (64-bit alignment even
  on 32-bit platforms), and for the sizes the mbuf header, packet header,
  and m_ext structure.

- Document that these assertions exist in comments in mbuf.h.

This change is not intended to cause (non-trivial) behavioural
differences, but is a precursor to further mbuf-allocator work.

Differential Revision:	https://reviews.freebsd.org/D1483
Reviewed by:	bz, gnn, np, glebius ("go ahead, I trust you")
Sponsored by:	EMC / Isilon Storage Division

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