mbuf: introduce a flag to indicate a control mbuf

mbuf: introduce a flag to indicate a control mbuf

Since the flags field is now 64-bits, we can allow one bit to be used to
indicate a control i.e. non-packet mbuf. Dedicate the high bit (bit 63)
for this purpose and add in a utility macro to test if a given mbuf has
the bit set or not.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>