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>

lib/packing.c

@@ -73,6 +73,7 @@ static void adjust_for_msb_right_quirk(u64 *to_write, int *box_start_bit,
  * @endbit: The index (in logical notation, compensated for quirks) where
  *	    the packed value ends within pbuf. Must be smaller than, or equal
  *	    to, startbit.
+ * @pbuflen: The length in bytes of the packed buffer pointed to by @pbuf.
  * @op: If PACK, then uval will be treated as const pointer and copied (packed)
  *	into pbuf, between startbit and endbit.
  *	If UNPACK, then pbuf will be treated as const pointer and the logical