Only alloc_mr_from_cache() needs order and can trivially compute it, so
lift it to the one call site and remove the NULL arguments.

Link: https://lore.kernel.org/r/20201026131936.1335664-6-leon@kernel.orgSigned-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>