RDMA/irdma: Check return value from ib_umem_find_best_pgsz

iwmr->page_size stores the return from ib_umem_find_best_pgsz and maybe
zero when used in ib_umem_num_dma_blocks thus causing a divide by zero
error.

Fix this by erroring out of irdma_reg_user when 0 is returned from
ib_umem_find_best_pgsz.

Link: https://lore.kernel.org/r/20210622175232.439-3-tatyana.e.nikolova@intel.com
Reported-by: coverity-bot <keescook+coverity-bot@chromium.org>
Addresses-Coverity-ID: 1505149 ("Integer handling issues")
Fixes: b48c24c2

 ("RDMA/irdma: Implement device supported verb APIs")
Signed-off-by: Shiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: Tatyana Nikolova <tatyana.e.nikolova@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>