smb3: prevent races updating CurrentMid

There was one place where we weren't locking CurrentMid, and although
likely to be safe since even without the lock since it is during
negotiate protocol, it is more consistent to lock it in this last remaining
place, and avoids confusing Coverity warning.

Addresses-Coverity: 1486665 ("Data race condition")
Signed-off-by: Steve French <stfrench@microsoft.com>