f2fs: refactor the hole reporting and allocation logic in f2fs_map_blocks

Add a is_hole local variable to figure out if the block number might need
allocation, and untangle to logic to report the hole or fill it with a
block allocation.

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