mm/damon/sysfs: simplify the variable 'pid' assignment operation

We can initialize the variable 'pid' with '-1' in pid_show() to simplify
the variable assignment operation and make the code more readable.

Link: https://lkml.kernel.org/r/1663060287-30201-3-git-send-email-kaixuxia@tencent.com


Signed-off-by: Kaixu Xia <kaixuxia@tencent.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>