vtnet: Fix regression introduced in r361944

For legacy devices that don't support MrgRxBuf (such as bhyve pre-r358180),
r361944 failed to update the receive handler to account for the additional
padding introduced by the unused num_buffers field that is now always present
in struct vtnet_rx_header. Thus, calculate the padding dynamically based on
vtnet_hdr_size.

PR: 247242
Reported by: thj
Tested by: thj