BUG_ON on kernel misbehavior on A100U2W driver

With broken Sparc64 IOMMU accounting, the kernel submits larger requests
then allowed.  Better to crash on BUG than corrupt memory.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Acked-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>