iommu sg: add IOMMU helper functions for the free area management

This adds IOMMU helper functions for the free area management.  These
functions take care of LLD's segment boundary limit for IOMMUs.  They would be
useful for IOMMUs that use bitmap for the free area management.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: Jeff Garzik <jeff@garzik.org>
Cc: James Bottomley <James.Bottomley@steeleye.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>