bond: Simplify bond_create.

Stop calling dev_get_by_name to see if the bond device already
exists.  register_netdevice already does that.

Stop calling bond_deinit if register_netdevice fails as bond_uninit
is guaranteed to be called if bond_init succeeds.

Signed-off-by: Eric W. Biederman <ebiederm@aristanetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>