net/ixgbe: eliminate mbuf write on rearm

net/ixgbe: eliminate mbuf write on rearm

With the mbuf rework, we now have 8 contiguous bytes to be rearmed in the
mbuf just before the 8-bytes of olflags. If we don't do the rearm write
inside the descriptor ring replenishment function, and delay it to
receiving the packet, we can do a single 16B write inside the RX function
to set both the rearm data, and the flags together.

Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>