btrfs: push struct writeback_control into start_delalloc_inodes

btrfs: push struct writeback_control into start_delalloc_inodes

In preparation for changing the filemap_fdatawrite_wbc API to not expose
the writeback_control to the callers, push the wbc declaration next to
the filemap_fdatawrite_wbc call and just pass the nr_to_write value to
start_delalloc_inodes.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20251024080431.324236-6-hch@lst.de
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>