RDMA/umem: Use simpler logic for ib_umem_find_best_pgsz()

The calculation in rdma_find_pg_bit() is fairly complicated, and the
function is never called anywhere else. Inline a simpler version into
ib_umem_find_best_pgsz()

Link: https://lore.kernel.org/r/3-v2-270386b7e60b+28f4-umem_1_jgg@nvidia.com


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