sysfs: turn WARN() into pr_warn()

It's not good to crash the machine if panic_on_warn() is set just
because someone made a stupid mistake of trying to create a sysfs file
with the same name of an existing one.  This makes the automated testing
tools a lot harder to find the real bugs in the kernel.

So just print a warning out and dump the stack to get the attention of
the developer that they did something foolish.  Then keep on trucking,
as this should not be a fatal error at all.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>