f2fs: open code allocate_segment_by_default

allocate_segment_by_default has just two callers, which use very
different code pathes inside it based on the force paramter.  Just
open code the logic in the two callers using a new helper to decided
if a new segment should be allocated.

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