net: Make the copy length in af_packet sockopt handler unsigned

fixed upstream in commit b7058842 in a different way

The length of the to-copy data structure is currently stored in
a signed integer. However many comparisons are done with sizeof(..)
which is unsigned. It's more suitable for this variable to be unsigned
to make these comparisons more naturally right.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>