A simple typo fix in the nl error message (fbd -> fdb).

CC: David Ahern <dsahern@gmail.com>
Fixes: 8c6e137f ("rtnetlink: Update rtnl_fdb_dump for strict data checking")
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>