writeback: move wb_wakeup_delayed defination to fs-writeback.c

The wb_wakeup_delayed is only used in fs-writeback.c. Move it to
fs-writeback.c after defination of wb_wakeup and make it static.

Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Link: https://lore.kernel.org/r/20240118203339.764093-1-shikemeng@huaweicloud.com


Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>