cifs: is_network_name_deleted should return a bool

Currently, is_network_name_deleted and it's implementations
do not return anything if the network name did get deleted.
So the function doesn't fully achieve what it advertizes.

Changed the function to return a bool instead. It will now
return true if the error returned is STATUS_NETWORK_NAME_DELETED
and the share (tree id) was found to be connected. It returns
false otherwise.

Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Acked-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>