selftests/damon/_chk_dependency: get debugfs mount point from /proc/mounts

selftests/damon/_chk_dependency: get debugfs mount point from /proc/mounts

DAMON debugfs selftests dependency checker assumes debugfs would be
mounted at /sys/kernel/debug.  That would be ok for many cases, but some
systems might mounted the file system on some different places.  Parse the
real mount point using /proc/mounts file.

Link: https://lkml.kernel.org/r/20240207203134.69976-9-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>