mm/page-writeback: raise wb_thresh to prevent write blocking with strictlimit

With the strictlimit flag, wb_thresh acts as a hard limit in
balance_dirty_pages() and wb_position_ratio().  When device write
operations are inactive, wb_thresh can drop to 0, causing writes to be
blocked.  The issue occasionally occurs in fuse fs, particularly with
network backends, the write thread is blocked frequently during a period. 
To address it, this patch raises the minimum wb_thresh to a controllable
level, similar to the non-strictlimit case.

Link: https://lkml.kernel.org/r/20241023100032.62952-1-jimzhao.ai@gmail.com


Signed-off-by: Jim Zhao <jimzhao.ai@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>