smb: handle file transactions post-GAP

smb: handle file transactions post-GAP

After a GAP all normal transactions are closed. File transactions
are left open as they can handle GAPs in principle. However, the
GAP might have contained the closing of a file and therefore it
may remain active until the end of the flow.

This patch introduces a time based heuristic for these transactions.
After the GAP all file transactions are stamped with the current
timestamp. If 60 seconds later a file has seen no update, its marked
as closed.

This is meant to fix resource starvation issues observed in long
running SMB sessions where packet loss was causing GAPs.