Mark ipoib device as initialized on device open.

Set the IPOIB_FLAG_INITIALIZED on dev_open and clear it on dev_stop to
avoid a race between ipoib load and the underlying device driver.

The device module must dispatch the IB_EVENT_PORT_ACTIVE event before ipoib
module is loaded. Otherwise, the flush will fail since no one set the
IPOIB_FLAG_INITIALIZED.

Submitted by: Slava Shwartsman <slavash@mellanox.com>
Sponsored by: Mellanox Technologies
MFC after: 1 week