dma-debug: fix overlap detection

dma-debug: fix overlap detection

Commit 0abdd7a81b7e ("dma-debug: introduce debug_dma_assert_idle()") was
reworked to expand the overlap counter to the full range expressable by
3 tag bits, but it has a thinko in treating the overlap counter as a
pure reference count for the entry.

Instead of deleting when the reference-count drops to zero, we need to
delete when the overlap-count drops below zero.  Also, when detecting
overflow we can just test the overlap-count > MAX rather than applying
special meaning to 0.

Regression report available here:
http://marc.info/?l=linux-netdev&m=139073373932386&w=2

This patch, now tested on the original net_dma case, sees the expected
handful of reports before the eventual data corruption occurs.

Signed-off-by: Dan Williams <[email protected]>
Reported-by: Sander Eikelenboom <[email protected]>
Cc: Francois Romieu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>