f2fs: Pass a folio to f2fs_submit_merged_write_cond()

Most callers pass NULL, and the one that passes a page already has a
folio.  Also convert __submit_merged_write_cond() to take a folio.

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