pvcalls-front: introduce a per sock_mapping refcount

Introduce a per sock_mapping refcount, in addition to the existing
global refcount. Thanks to the sock_mapping refcount, we can safely wait
for it to be 1 in pvcalls_front_release before freeing an active socket,
instead of waiting for the global refcount to be 1.

Signed-off-by: Stefano Stabellini <[email protected]>
Acked-by: Juergen Gross <[email protected]>
Signed-off-by: Juergen Gross <[email protected]>