swiotlb: save io_tlb_used to local variable before leaving critical section

When swiotlb is full, the kernel would print io_tlb_used. However, the
result might be inaccurate at that time because we have left the critical
section protected by spinlock.

Therefore, we backup the io_tlb_used into local variable before leaving
critical section.

Fixes: 83ca2594 ("swiotlb: dump used and total slots when swiotlb buffer is full")
Suggested-by: Håkon Bugge <haakon.bugge@oracle.com>
Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>