Display number of credits available

In debugging smb3, it is useful to display the number
of credits available, so we can see when the server has not granted
sufficient operations for the client to make progress, or alternatively
the client has requested too many credits (as we saw in a recent bug)
so we can compare with the number of credits the server thinks
we have.

Add a /proc/fs/cifs/DebugData line to display the client view
on how many credits are available.

Signed-off-by: Steve French <steve.french@primarydata.com>
Reported-by: Germano Percossi <germano.percossi@citrix.com>
CC: Stable <stable@vger.kernel.org>