mbuf: clarify comment on adding Tx offload flags

mbuf: clarify comment on adding Tx offload flags

The comment for TX offload flags stated that those flags started at bit
55 and then were added to the right of that, leaving 8 bits reserved for
generic mbuf (i.e. non-offload) use. This comment may not have been
clear as 5 of the 8 flags which were reserved have now been used for TX
offloads.
This patch:
* updates the description so that it now reflects reality that
only three flags are available for generic mbuf use
* reserved the final generic flag so that it can't be taken over for TX
offload in future
* clarifies the comment for TX flags to indicate that they should be
counting downwards not upwards.

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