rtnetlink: Fix inverted check in ndo_dflt_fdb_del()

[ Upstream commit 645359930231d5e78fd3296a38b98c1a658a7ade ]

Fix inverted check when deleting an fdb entry.

Signed-off-by: Sridhar Samudrala <sri@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>