CHROMIUM: security: bug fix: Return zero on success for csm_sendmsg

The csm_sendmsg returns the number of bytes sent but the intent was to
return zero on success. It translates to additional errors in dmesg and
incorrect number of bytes returned on sysfs container write (unused).

BUG=b:126255140
TEST=Checked no error is logged on process creation/exit with dummy
backend.

Signed-off-by: Thomas Garnier <thgarnie@chromium.org>
Change-Id: I8a0af1fe0da504c9070dd6b6a063ec35c4ea2377
Reviewed-on: https://chromium-review.googlesource.com/1487379
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>
(cherry picked from commit cb36dc523182ff3f726de608f38da76c354798aa)
Reviewed-on: https://chromium-review.googlesource.com/c/1491691