Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Give (*ext_free) an int return value allowing for very sophisticated
external mbuf buffer management capabilities in the future.

For now only EXT_FREE_OK is defined with current legacy behavior.

Sponsored by: The FreeBSD Foundation

Add an mbuf pointer parameter to (*ext_free) to give the external
free function access to the mbuf the external memory was attached
to.

Mechanically adjust all users to include the mbuf parameter.

This fixes a long standing annoyance for external free functions.
Before one had to sacrifice one of the argument pointers for this.

Sponsored by: The FreeBSD Foundation

/* -> /*- for license, minor formatting changes

Add a facility for devices, specifically network interfaces, that require
large to huge amounts of small or medium sized receive buffers. The problem
with these situations is that they eat up the available DMA address space
very quickly when using mbufs or even mbuf clusters. Additionally this
facility provides a direct mapping between 32-bit integers and these buffers.
This is needed for devices originally designed for 32-bit systems. Ususally
the virtual address of the buffer is used as a handle to find the buffer as
soon as it is returned by the card. This does not work for 64-bit machines
and hence this mapping is needed.