swiotlb: add debugfs to track swiotlb buffer usage

swiotlb: add debugfs to track swiotlb buffer usage

The device driver will not be able to do dma operations once swiotlb buffer
is full, either because the driver is using so many IO TLB blocks inflight,
or because there is memory leak issue in device driver. To export the
swiotlb buffer usage via debugfs would help the user estimate the size of
swiotlb buffer to pre-allocate or analyze device driver memory leak issue.

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 71602fe6d4e9291af105adfef8e893b57c735906)
Signed-off-by: Brian Maly <brian.maly@oracle.com>
Conflicts:
  debugfs_create_ulong() is not available. debugfs_create_file() is used instead.

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>