Fix r273112: do not turn DROP_EN by default.

Due to adapter->hw.fc.requested_mode is filled with default value
after ixgbe_initialize_receive_units(), this leads to enabling
DROP_EN in most cases.

Tested by:	ae
MFC after:	1 week
Revert r274585: rte lock is properly destroyed in uma dtor callback.

Pointed by:	glebius