RDMA/core: Remove unnecessary statements

The rdma_zalloc_drv_obj() in __ib_alloc_pd() would zero pd, it unnecessary
add NULL to the object in struct pd.

The uverbs_free_pd() already return busy if pd->usecnt is true, there is
no need to add a warning.

Link: https://lore.kernel.org/r/20220223074901.201506-1-yajun.deng@linux.dev


Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>