This was being performed to ensure that a complete packet was held in
contiguous memory, prior to parsing the packet. However, the source buffer
is typically large enough that the packet was already contiguous, so it is
beneficial to return the packet by reference in most cases.

         Before          After
         Mean   StdDev   Mean   StdDev  Change
memcpy    720.7  32.7     649.8  25.1   +10.9%
Overall  2372.7  46.1    2291.7  21.8    +3.5%
Signed-off-by: Martin Storsjö <martin@martin.st>