This adds EEXIST to distinguish between the following return values:
0: 	nobody was registered, registration successful
EEXIST:	the exact same handler was already registered, no registration
	required
EBUSY:	somebody else is registered, registration unsuccessful.
Signed-off-by: NHarald Welte <laforge@netfilter.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>