f2fs: return immediately after submitting the specified folio in __submit_merged_writ...

f2fs: return immediately after submitting the specified folio in __submit_merged_write_cond

f2fs_folio_wait_writeback ensures the folio write is submitted to the
block layer via __submit_merged_write_cond, then waits for the folio to
complete. Other I/O submissions are irrelevant to
f2fs_folio_wait_writeback. Thus, if the folio write bio is already
submitted, the function can return immediately. This patch adds a
writeback parameter to __submit_merged_write_cond(), which signals an
immediate return after submitting the target folio, and waitting
writeback can use this parameter.

Signed-off-by: Yongpeng Yang <yangyongpeng@xiaomi.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>