cifs: enable extended stats by default

CONFIG_CIFS_STATS2 can be very useful since it shows
latencies by command, and allows enabling the slow response
dynamic tracepoint which can be useful to identify
performance problems.

For example:

Total time spent processing by command. Time units are jiffies (1000 per second)
  SMB3 CMD	Number	Total Time	Fastest	Slowest
  --------	------	----------	-------	-------
  0		1	2		2	2
  1		2	6		2	4
  2		0	0		0	0
  3		4	11		2	4
  4		2	16		5	11
  5		4546	34104		2	487
  6		4421	32901		2	487
  7		0	0		0	0
  8		695	2781		2	39
  9		391	1708		2	27
  10		0	0		0	0
  11		4	6		1	2
  12		0	0		0	0
  13		0	0		0	0
  14		3887	17696		0	128
  15		0	0		0	0
  16		1471	9950		1	487
  17		169	2695		9	116
  18		80	381		2	10
  1		2	6		2	4
  2		0	0		0	0
  3		4	11		2	4
  4		2	16		5	11
  5		4546	34104		2	487
  6		4421	32901		2	487
  7		0	0		0	0
  8		695	2781		2	39
  9		391	1708		2	27
  10		0	0		0	0
  11		4	6		1	2
  12		0	0		0	0
  13		0	0		0	0
  14		3887	17696		0	128
  15		0	0		0	0
  16		1471	9950		1	487
  17		169	2695		9	116
  18		80	381		2	10

Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>