mm/backing-dev.c: remove recently-added WARN_ON()

mm/backing-dev.c: remove recently-added WARN_ON()

commit 69fc208be5b7eb18d22d1eca185b201400fd5ffc upstream.

On second thoughts, this is just going to disturb people while telling us
things which we already knew.

Cc: Peter Korsgaard <jacmet@sunsite.dk>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>