smb3: add tracepoint for slow responses

If responses take longer than one second from the server,
we can optionally log them to dmesg in current cifs.ko code
(CONFIG_CIFS_STATS2 must be configured and a
/proc/fs/cifs/cifsFYI flag must be set), but can be more useful
to log these via ftrace (tracepoint is smb3_slow_rsp) which
is easier and more granular (still requires CONFIG_CIFS_STATS2
to be configured in the build though).

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