fault-inject: simplify access check for fail-nth

fault-inject: simplify access check for fail-nth

The fail-nth file is created with 0666 and the access is permitted if
and only if the task is current.

This file is owned by the currnet user.  So we can create it with 0644
and allow the owner to write it.  This enables to watch the status of
task->fail_nth from another processes.

[[email protected]: don't convert unsigned type value as signed int]
Link: http://lkml.kernel.org/r/[email protected]
[[email protected]: avoid unwanted data race to task->fail_nth]
Link: http://lkml.kernel.org/r/[email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Akinobu Mita <[email protected]>
Acked-by: Dmitry Vyukov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>