LAPDm code pulls both 'l1h' and 'l2h' of msgb
In some cases, it's required to keep some data before the actual MAC-block, e.g. in order to indicate
the FDMA/TDMA info (frame number, ARFCN, etc.) to the upper layers, but the current implementation
doesn't allow this, because it calls msgb_pull_to_l3() stripping all headers. In other words,
when a message buffer is being passed through the current LAPDm code, everything before the
data frame is getting lost.