mm/damon/sysfs: eliminate potential uninitialized variable warning

[ Upstream commit 85c2ceaa

 ]

The "err" variable is not initialized if damon_target_has_pid(ctx) is
false and sys_target->regions->nr is zero.

Link: https://lkml.kernel.org/r/739e6aaf-a634-4e33-98a8-16546379ec9f@moroto.mountain
Fixes: 0bcd216c4741 ("mm/damon/sysfs: update monitoring target regions for online input commit")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>