page_shift is used to calculate the page size, it's always non-negative,
and should be in type of unsigned.

Link: https://lore.kernel.org/r/1589982799-28728-7-git-send-email-liweihang@huawei.comSigned-off-by: Weihang Li <liweihang@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>