alpha: IOMMU had better access to the free space bitmap at only one place

iommu_arena_find_pages duplicates the code to access to the bitmap for free
space management.  This patch convert the IOMMU code to have only one place to
access the bitmap, in the popular way that other IOMMUs (e.g.  POWER and
SPARC) do.

This patch is preparation for modifications to fix the IOMMU segment boundary
problem.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>