f2fs: simplify return value handling in f2fs_fsync_node_pages

Always assign ret where the error happens, and jump to out instead
of multiple loop exit conditions to prepare for changes in the
__write_node_folio calling convention.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>