IB/core: Make ib_alloc_device init the kobject

This gets rid of the weird in-between state where struct ib_device
was allocated but the kobject didn't work.

Consequently ib_device_release is now guaranteed to be called in
all situations and we needn't duplicate its kfrees on error paths.

Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>