swiotlb: consolidate rounding up default_nslabs

default_nslabs are rounded up in two cases with exactly same comments.
Add a simple wrapper to reduce duplicate code/comments. It is preparatory
to adding more logics into the round-up.

No functional change intended.

Signed-off-by: Chao Gao <chao.gao@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>