Add new M_START() and M_SIZE() macros mbufs that return a pointer to the start of an mbuf's storage, and return its size (in contrast to m_data and m_len that are with respect to data in the buffer). Rewrite M_LEADINGSPACE() and M_TRAILINGSPACE() in terms of M_START() and M_SIZE().
This is done as we currently have many instances of using mbuf flags to generate pointers or lengths for internal storage in header and regular mbufs, as well as to external storage. Rather than replicate this logic throughout the network stack, centralising the implementation will make it easier for us to refine mbuf storage. It should also help reduce bugs by limiting the amount of mbuf-type-specific pointer arithmetic. Followup changes will propagate use of the macros throughout the stack.
M_SIZE() conflicts with one macro in the Chelsio driver; rename that macro in a slightly unsatisfying way to eliminate the collision.
MFC after: 3 days
Sponsored by: EMC / Isilon Storage Division
Reviewed by: bz, glebius