mtr_umem_page_count() does the same thing, replace it with the core code.

Also, ib_umem_find_best_pgsz() should always be called to check that the
umem meets the page_size requirement. If there is a limited set of
page_sizes that work it the pgsz_bitmap should be set to that set. 0 is a
failure and the umem cannot be used.

Lightly tidy the control flow to implement this flow properly.

Link: https://lore.kernel.org/r/12-v2-270386b7e60b+28f4-umem_1_jgg@nvidia.comAcked-by: NWeihang Li <liweihang@huawei.com>
Signed-off-by: NJason Gunthorpe <jgg@nvidia.com>