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

mainline inclusion
from mainline-v6.2-rc6
commit f1c71c28
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I6MRUG

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f1c71c2825218dc8b35c04ab439fdf3d32778c7c

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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>
Signed-off-by: Ze Zuo <zuoze1@huawei.com>