iommufd/iova_bitmap: Consolidate iova_bitmap_set exit conditionals

iommufd/iova_bitmap: Consolidate iova_bitmap_set exit conditionals

There's no need to have two conditionals when they are closely tied
together. Move the setting of bitmap::set_ahead_length after it checks for
::pages array out of bounds access.

Link: https://lore.kernel.org/r/20240627110105.62325-10-joao.m.martins@oracle.com
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Tested-by: Matt Ochs <mochs@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>