IB/ipoib: Move all uninit code into ndo_uninit

Currently uninit is sometimes done twice in error flows, and is sprinkled
a bit all over the place.

Improve the clarity of the design by moving all uninit only into
ndo_uinit.

Some duplication is removed:
 - Sometimes IPOIB_STOP_NEIGH_GC was done before unregister, but
   this duplicates the process in ipoib_neigh_hash_init
 - Flushing priv->wq was sometimes done before unregister,
   but that duplicates what has been done in ndo_uninit

Uniniting the IB event queue must remain before unregister_netdev as it
requires the RTNL lock to be dropped, this is moved to a helper to make
that flow really clear and remove some duplication in error flows.

If register_netdev fails (and ndo_init is NULL) then it almost always
calls ndo_uninit, which lets us remove all the extra code from the error
unwinds. The next patch in the series will close the 'almost always' hole
by pairing a proper ndo_init with ndo_uninit.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>