iomap: add dirty page control to iomap_zero_iter

iomap: add dirty page control to iomap_zero_iter

This patch prepares the iomap framework for exFAT's upcoming migration to
iomap. During testing of the exFAT iomap branch with xfstests generic/299 on
a VM with 8GB RAM and a 40GB disk, system unresponsiveness was observed.

iomap_zero_iter() lacked dirty page throttling, which could cause memory
pressure when exFAT's valid_size mechanism triggers large-scale zeroing
operations during writes beyond valid_size.

Align iomap_zero_iter() with iomap_write_iter() by adding
balance_dirty_pages_ratelimited() to throttle dirty page generation during
large zeroing operations

Signed-off-by: Chi Zhiling <chizhiling@kylinos.cn>
Link: https://patch.msgid.link/20260511094007.728011-1-chizhiling@163.com
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>