fs: Remove aop flags parameter from cont_write_begin()

There are no more aop flags left, so remove the parameter.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>