batman-adv: tt: prevent TVLV OOB check overflow

batman-adv: tt: prevent TVLV OOB check overflow

A TT unicast TVLV contains the number of VLANs stored in it. This number is
an u16 and gets multiplied by the size of the struct
batadv_tvlv_tt_vlan_data (8 bytes). The size can therefore overflow the u16
used to store the tt_vlan_len. All additional safety checks to prevent
out-of-bounds access of the TVLV buffer are invalid due to this overflow.

Using size_t prevents this overflow and ensures that the safety checks
compare against the actual buffer requirements.

Cc: stable@vger.kernel.org
Fixes: 7ea7b4a14275 ("batman-adv: make the TT CRC logic VLAN specific")
Signed-off-by: Sven Eckelmann <sven@narfation.org>