debugfs: return error values, not NULL

debugfs: return error values, not NULL

When an error happens, debugfs should return an error pointer value, not
NULL.  This will prevent the totally theoretical error where a debugfs
call fails due to lack of memory, returning NULL, and that dentry value
is then passed to another debugfs call, which would end up succeeding,
creating a file at the root of the debugfs tree, but would then be
impossible to remove (because you can not remove the directory NULL).

So, to make everyone happy, always return errors, this makes the users
of debugfs much simpler (they do not have to ever check the return
value), and everyone can rest easy.

Reported-by: Gary R Hook <[email protected]>
Reported-by: Heiko Carstens <[email protected]>
Reported-by: Masami Hiramatsu <[email protected]>
Reported-by: Michal Hocko <[email protected]>
Reported-by: Sebastian Andrzej Siewior <[email protected]>
Reported-by: Ulf Hansson <[email protected]>
Reviewed-by: Masami Hiramatsu <[email protected]>
Reviewed-by: Sebastian Andrzej Siewior <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>