[IA64] fix SBA IOMMU to handle allocation failure properly

It's possible that SBA IOMMU might fail to find I/O space under heavy
I/Os.  SBA IOMMU panics on allocation failure but it shouldn't; drivers
can handle the failure.  The majority of other IOMMU drivers don't panic
on allocation failure.

This patch fixes SBA IOMMU path to handle allocation failure properly.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>