cifs: fix clearing stats for fastest execution of each smb2 command

The code to clear the 'fastest_cmd' statistics has a typo that
repeatedly clears the stat for cmd 0, rather than iterating
through each cmd.  Fix the typo (0->i).

Fixes: 433b8dd7672be ("SMB3: Track total time spent on roundtrips for each SMB3 command")
Signed-off-by: Frank Sorenson <sorenson@redhat.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>