RDMA/usnic: Clean up usnic_ib_alloc_pd()

Remove the unnecessary "umem_pd" variable.  And usnic_uiom_alloc_pd()
never returns NULL so remove the NULL check.

Link: https://lore.kernel.org/r/20211118113924.GH1147@kili


Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>