RDMA/i40iw: Use ib_umem_num_dma_pages()

If ib_umem_find_best_pgsz() returns > PAGE_SIZE then the equation here is
not correct. 'start' should be 'virt'. Change it to use the core code for
page_num and the canonical calculation of page_shift.

Fixes: eb52c033 ("RDMA/i40iw: Use core helpers to get aligned DMA address within a supported page size")
Link: https://lore.kernel.org/r/8-v2-270386b7e60b+28f4-umem_1_jgg@nvidia.com


Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>