smb3: add dynamic trace point for smb3_cmd_enter

Add tracepoint before sending an SMB3 command on the wire (ie add
an smb3_cmd_enter tracepoint). This allows us to look in much
more detail at response times (between request and response).

Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>