swiotlb: checking whether swiotlb buffer is full with io_tlb_used

swiotlb: checking whether swiotlb buffer is full with io_tlb_used

This patch uses io_tlb_used to help check whether swiotlb buffer is full.
io_tlb_used is no longer used for only debugfs. It is also used to help
optimize swiotlb_tbl_map_single().

Suggested-by: Joe Jin <joe.jin@oracle.com>
Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Orabug: 29582587
(cherry picked from commit 60513ed06a41049768a6875229b025b6e726e148)

Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>