smb3: add mount option to allow RW caching of share accessed by only 1 client

If a share is known to be only to be accessed by one client, we
can aggressively cache writes not just reads to it.

Add "cache=" option (cache=singleclient) for mounting read write shares
(that will not be read or written to from other clients while we have
it mounted) in order to improve performance.

Signed-off-by: Steve French <stfrench@microsoft.com>