RDMA/umem: Avoid undefined behavior of rounddown_pow_of_two()

RDMA/umem: Avoid undefined behavior of rounddown_pow_of_two()

rounddown_pow_of_two() is undefined when the input is 0. Therefore we need
to avoid it in ib_umem_find_best_pgsz and return 0.  Otherwise, it could
result in not rejecting an invalid page size which eventually causes a
kernel oops due to the logical inconsistency.

Fixes: 3361c29e9279 ("RDMA/umem: Use simpler logic for ib_umem_find_best_pgsz()")
Link: https://lore.kernel.org/r/20210113121703.559778-2-leon@kernel.org
Signed-off-by: Aharon Landau <aharonl@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>