packet: Fix undefined behavior in bit shift

Shifting signed 32-bit value by 31 bits is undefined.  Changing most
significant bit to unsigned.

Changes included in v2:
  - use subsystem specific subject lines
  - CC required mailing lists

Signed-off-by: Jiunn Chang <c0d1n61at3@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>