Testing the msgb API
Buffer: 
put(4) -> data+0
Buffer: [L1]> 00 00 00 00 
put(4) -> data+4
Buffer: [L1]> 00 00 00 00 [L2]> 00 00 00 00 
put(4) -> data+8
Buffer: [L1]> 00 00 00 00 [L2]> 00 00 00 00 [L3]> 00 00 00 00 
put(4) -> data+12
Buffer: [L1]> 00 00 00 00 [L2]> 00 00 00 00 [L3]> 00 00 00 00 [L4]> 00 00 00 00 
push(4) -> data+0
Buffer: 00 00 00 00 [L1]> 00 00 00 00 [L2]> 00 00 00 00 [L3]> 00 00 00 00 [L4]> 00 00 00 00 
trim(16) -> 0
Buffer: 00 00 00 00 [L1]> 00 00 00 00 [L2]> 00 00 00 00 [L3]> 00 00 00 00 [L4]> 
get(4) -> data+12
Buffer: 00 00 00 00 [L1]> 00 00 00 00 [L2]> 00 00 00 00 [L3]> (L4=tail+4) 
Test msgb_hexdump
Buffer: (L1=data-124) 00 00 00 00 00 00 00 00 [L2]> 00 00 00 00 [L3]> (L4=tail+4) 
Buffer: (L1=data-124) 00 00 00 00 00 00 00 00 [L2]> (L3+8) 00 00 00 00 (L4=tail+4) 
Buffer: (L1=data-124) 00 00 00 00 00 00 00 00 [L2]> 00 00 00 00 (L3 out of range) (L4=tail+4) 
Success.