lib: packing: add documentation for pbuflen argument

Fixes sparse warning:

Function parameter or member 'pbuflen' not described in 'packing'

Fixes: 554aae35 ("lib: Add support for generic packing operations")
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>