ixgbe: keep only non-zero initializer in mbuf definition

ixgbe: keep only non-zero initializer in mbuf definition

Since all unspecified fields in an initializer are assumed to be zero we
can simplify the empty mbuf definition in the vector driver to only use
the fields that are non-zero, i.e. just nb_segs = 1. This makes things
shorter and means that the structure doesn't need as many updates for
other fields being renamed or moved.

The variable itself is never modified and only used by a single function
so it can be made const and local to the using function.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>