selftests/damon/_damon_sysfs: find sysfs mount point from /proc/mounts

_damon_sysfs.py assumes sysfs is mounted at /sys.  In some systems, that
might not be true.  Find the mount point from /proc/mounts file content.

Link: https://lkml.kernel.org/r/20240503180318.72798-4-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>