Linux 6.18: replace write_cache_pages()

Linux 6.18 removed write_cache_pages() without a usable replacement.
Here we implement a minimal zpl_write_cache_pages() that find the dirty
pages within the mapping, gets them into the expected state and hands
them off to zfs_putpage(), which handles the rest.

Sponsored-by: https://despairlabs.com/sponsor/
Signed-off-by: Rob Norris <robn@despairlabs.com>