bonding: convert curr_slave_lock to a spinlock and rename it

curr_slave_lock is now a misleading name, a much better name is
mode_lock as it'll be used for each mode's purposes and it's no longer
necessary to use a rwlock, a simple spinlock is enough.

Suggested-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>