intel-iommu: Check for an RMRR which ends before it starts.

Some HP BIOSes report an RMRR region (a region which needs a 1:1 mapping
in the IOMMU for a given device) which has an end address lower than its
start address. Detect that and warn, rather than triggering the
BUG() in dma_pte_clear_range().

Cc: stable@kernel.org
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>