IPoIB: Fix crash in dev_open error flow

If napi has never been enabled when calling ipoib_ib_dev_stop, a
kernel crash occurs, because the verbs layer completion handler
(ipoib_ib_completion) calls napi_schedule unconditionally.

If the napi structure passed in the napi_schedule call has not
been initialized, napi will crash.

The cleanest solution is to simply enable napi before calling
ipoib_ib_dev_stop in the dev_open error flow. (dev_stop then
immediately disables napi).

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Erez Shitrit <erezsh@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>