swiotlb: Move alloc_size to swiotlb_find_slots

Rename find_slots to swiotlb_find_slots and move the maintenance of
alloc_size to it for better code reusability later.

Signed-off-by: Claire Chang <tientzu@chromium.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: Stefano Stabellini <sstabellini@kernel.org>
Tested-by: Will Deacon <will@kernel.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>