mm: start background writeback based on per-wb threshold for strictlimit BDIs

The proactive nr_dirty > gdtc->bg_thresh check in balance_dirty_pages()
only checks the global dirty threshold to start background writeback while
the writer is still free-running, but for strictlimit BDIs (eg fuse), the
per-wb dirty count can exceed the per-wb background threshold while the
global threshold is not yet exceeded, so background writeback for this
case never gets proactively started.

Add a per-wb threshold check for strictlimit BDIs so that background
writeback is started when wb_dirty exceeds wb_bg_thresh, which drains
dirty pages before the writer hits the throttle wall, matching the
proactive behavior that the global check provides for non-strictlimit
BDIs.

fio runs on fuse show about a 3-4% improvement in perf for buffered
writes:
fio --name=writeback_test --ioengine=psync --rw=write --bs=128k \
    --size=2G --numjobs=4 --ramp_time=10 --runtime=20 \
    --time_based --group_reporting=1 --direct=0

Link: https://lore.kernel.org/20260326234629.840938-2-joannelkoong@gmail.com
Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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