kernel task cleanup

In case of cleanup tasks in Accelio kernel need to unmap the dma
in pre put and not in destory as users messages are already
returned to the application (via flush).

This is partial solution to issue 401086 named
"NULL pointer dereference when unloading rdma module after xnbd module"

Issue: 401086

Change-Id: I11b6323d8a9fb240293f72b148a95a863d200676
Signed-off-by: Shlomo Pongratz <shlomop@mellanox.com>