selftests/damon: test debugfs file reads/writes with huge count

selftests/damon: test debugfs file reads/writes with huge count

commit b4a002889d24979295ed3c2bf1d5fcfb3901026a upstream.

DAMON debugfs interface users were able to trigger warning by writing
some files with arbitrarily large 'count' parameter.  The issue is fixed
with commit db7a347b26fe ("mm/damon/dbgfs: use '__GFP_NOWARN' for
user-specified size buffer allocation").  This commit adds a test case
for the issue in DAMON selftests to avoid future regressions.

Link: https://lkml.kernel.org/r/20211201150440.1088-11-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Brendan Higgins <brendanhiggins@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>