The mbuf type is an enumerator with only a handful of types in use and
thus reduced from int to 8bits allowing for 255 types to be specified.
Only 5 types have been in use for a long time.
The flags field gets the remaining 24 bits with 12 bits for global
persistent flags and 12 bits for protocol/layer specific overlays.
Some of the global flags/functionality can be moved to the csum_flags
or ext_flags bits in the future.
MT_VENDOR[1-4] and MT_EXP[1-4] types for vendor-internal and
experimental local mapping are added.
The size of m_hdr shrinks from 24/40 to 20/32bytes (32/64bit architectures).
Sponsored by: The FreeBSD Foundation
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