[BRIDGE]: Fix locking of set path cost.

This change goes with earlier change to get rid of
work queue for path cost. Now stp_set_path_cost does its own
locking. This is to allow it to call br_path_cost() which calls
ethtool interfaces (might sleep).

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>