RDMA/efa: 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: 40ddb3f0

 ("RDMA/efa: Use API to get contiguous memory blocks aligned to device supported page size")
Link: https://lore.kernel.org/r/7-v2-270386b7e60b+28f4-umem_1_jgg@nvidia.com
Tested-by: Gal Pressman <galpress@amazon.com>
Acked-by: Gal Pressman <galpress@amazon.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>