f2fs: clean up the force parameter in __submit_merged_write_cond()

f2fs: clean up the force parameter in __submit_merged_write_cond()

The force parameter in __submit_merged_write_cond is redundant, where
`force == true` implies `inode == NULL && folio == NULL && ino == 0` is
true, and `force == false` implies `inode != NULL || folio != NULL ||
ino != 0` is true. Thus, this patch replaces the force parameter with
a stack variable force.

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