mm: remove the for_reclaim field from struct writeback_control

mm: remove the for_reclaim field from struct writeback_control

This field is now only set to one in the i915 gem code that only calls
writeback_iter on it, which ignores the flag.  All other checks are thuse
dead code and the field can be removed.

Link: https://lkml.kernel.org/r/20250610054959.2057526-7-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>