cifs: reconnect work should have reference on server struct

The delayed work for reconnect takes server struct
as a parameter. But it does so without holding a ref
to it. Normally, this may not show a problem as
the reconnect work is only cancelled on umount.

However, since we now plan to support scaling down of
channels, and the scale down can happen from reconnect
work itself, we need to fix it.

This change takes a reference on the server struct
before it is passed to the delayed work. And drops
the reference in the delayed work itself. Or if
the delayed work is successfully cancelled, by the
process that cancels it.

Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>