ipoib: Fix for accessing uninitialized pointers and freed memory during attach and detach.

Call infiniband_ifdetach() early to stop ifioctl(9) calls from user-space
during device removal. Also make sure that ifioctl(9) calls are blocked from
executing until the device is fully initialized. Ideally we would delay the
infiniband_ifattach() call, but because part of the initialization is to update
the link level address, that is not possible without more significant changes.

MFC after: 1 week
Reviewed by: kib
Sponsored by: Mellanox Technologies // NVIDIA Networking