f2fs: Remove check for ->writepage

We're almost able to remove a_ops->writepage.  This check is unnecessary
as we'll never call into __f2fs_write_data_pages() for character
devices.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>