IB/ipoib: Get rid of IPOIB_FLAG_GOING_DOWN

This essentially duplicates the netdev's reg_state, so just use that
directly. The reg_state is updated under the rntl_lock, and all places
using GOING_DOWN already acquire the rtnl_lock so checking is safe.

Since the only place we use GOING_DOWN is for the parent device this
does not fix any bugs, but it is a step to tidy up the unregister flow
so that after later patches the flow is uniform and sane.

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