We've seen the in-flight count go into negative with some
internal stress testing in Microsoft.

Adding a WARN when this happens, in hope of understanding
why this happens when it happens.
Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Reviewed-by: Bharath SM <bharathsm@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>