bonding: fix locking in enslave failure path

In commit 3c5913b5 ("bonding:
primary_slave & curr_active_slave are not cleaned on enslave failure")
I didn't account for the use of curr_active_slave without curr_slave_lock
and since there are such users, we should hold bond->lock for writing while
setting it to NULL (in the NULL case we don't need the curr_slave_lock).
Keeping the bond lock as to avoid the extra release/acquire cycle.

Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>