IB/uverbs: Fix uverbs_attr_get_obj

The err pointer comes from uverbs_attr_get, not from the uobject member,
which does not store an ERR_PTR.

Fixes: be934cca ("IB/uverbs: Add device memory registration ioctl support")
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>