writeback: wbc_attach_fdatawrite_inode out of line

This allows exporting this high-level interface only while keeping
wbc_attach_and_unlock_inode private in fs-writeback.c and unexporting
__inode_attach_wb.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20241112054403.1470586-3-hch@lst.de


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