writeback: fix break condition

Commit dcf6a79d

 ("write-back: fix
nr_to_write counter") fixed nr_to_write counter, but didn't set the break
condition properly.

If nr_to_write == 0 after being decremented it will loop one more time
before setting done = 1 and breaking the loop.

[akpm@linux-foundation.org: coding-style fixes]
Cc: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Acked-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>