LU-13470 ptlrpc: return proper write count from ping_store

LU-13470 ptlrpc: return proper write count from ping_store

If we return 0 as a measure of success, that is actually taken
by userspace as "we wrote 0 bytes, but there's no error, so
they want us to retry". So tell them we ate all their data
instead to make them happy.

Interesting that when you do shell-redirect it enters the
retry loop, but badarea_io does not.

Change-Id: Ieb0f8bcd4f010ab4375ea6f65f6ea00248a236cb
Signed-off-by: Oleg Drokin <green@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/38304
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>