remove redefinition to vxlanhdr struct in vxlan dissector

[mmaatuq]

• vxlanhdr struct was redefined in vxlan dissector
• in this PR @utoni asked me to remove a redefinition of vxlanhdr struct.

[IvanNardi]

Instead of accessing the data byte-by-byte, I think it would be cleaner if you use the structure already defined in src/include/ndpi_typedefs.h (updating it, if you think so)

[mmaatuq]

I thought we could redefine the struct as per rfc-7348

struct ndpi_vxlanhdr{
    u_int8_t flags;
    u_int8_t reserved1[3];
    uint_8_t vni[3];
    uint_8_t reserved2;
};

after looking at some references, and how they defined rfc-7348 vxlanhdr
linux kernel

struct vxlanhdr 
   __be32 vx_flags;
   __be32 vx_vni;
};

wireshark
there is not direct definition for vxlanhdr but from the protocol dissector

struct vxlanhdr {
    u_int16_t flags;
    u_int16_t group_policy;
    u_int8_t vni[3];
    uint_8_t reserved;
};

The last definition is more convenient for nDPI (classification)

[sonarcloud]

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[IvanNardi]

@mmaatuq, thanks!