LU-13299 lnet: add "stats reset" to lnetctl

LU-13299 lnet: add "stats reset" to lnetctl

This new command resets stats shown by "lnetctl stats show". It could
be useful when debugging connectivity issues, by making easier the
process to detect the changes in stats from the clean state rather
than on top of historical values.

Signed-off-by: Cyril Bordage <cbordage@whamcloud.com>
Change-Id: I4195a862fa5e04d96ac4c2b1509b625c90fbb579
Reviewed-on: https://review.whamcloud.com/44150
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Amir Shehata <ashehata@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>