SMB3: Add mount parameter to allow user to override max credits

Add mount option "max_credits" to allow setting maximum SMB3
credits to any value from 10 to 64000 (default is 32000).
This can be useful to workaround servers with problems allocating
credits, or to throttle the client to use smaller amount of
simultaneous i/o or to workaround server performance issues.

Also adds a cap, so that even if the server granted us more than
65000 credits due to a server bug, we would not use that many.

Signed-off-by: Steve French <steve.french@primarydata.com>