iommufd: Prevent ALIGN() overflow

iommufd: Prevent ALIGN() overflow

When allocating IOVA the candidate range gets aligned to the target
alignment. If the range is close to ULONG_MAX then the ALIGN() can
wrap resulting in a corrupted iova.

Open code the ALIGN() using get_add_overflow() to prevent this.
This simplifies the checks as we don't need to check for length earlier
either.

Consolidate the two copies of this code under a single helper.

This bug would allow userspace to create a mapping that overlaps with some
other mapping or a reserved range.

Cc: stable@vger.kernel.org
Fixes: 51fe6141f0f6 ("iommufd: Data structure to provide IOVA to PFN mapping")
Reported-by: syzbot+c2f65e2801743ca64e08@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/r/685af644.a00a0220.2e5631.0094.GAE@google.com
Reviewed-by: Yi Liu <yi.l.liu@intel.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Link: https://patch.msgid.link/all/1-v1-7b4a16fc390b+10f4-iommufd_alloc_overflow_jgg@nvidia.com/
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>