swiotlb: dump used and total slots when swiotlb buffer is full

swiotlb: dump used and total slots when swiotlb buffer is full

So far the kernel only prints the requested size if swiotlb buffer if full.
It is not possible to know whether it is simply an out of buffer, or it is
because swiotlb cannot allocate buffer with the requested size due to
fragmentation.

As 'io_tlb_used' is available since commit 71602fe6d4e9 ("swiotlb: add
debugfs to track swiotlb buffer usage"), both 'io_tlb_used' and
'io_tlb_nslabs' are printed when swiotlb buffer is full.

Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Orabug: 29637525

(cherry picked from commit 83ca259489409a1fe8a83dad83a82f32174d4f31)
Signed-off-by: Brian Maly <brian.maly@oracle.com>
Conflicts:
    kernel/dma/swiotlb.c does not exist.

Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Reviewed-By: Joe Jin <joe.jin@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>