f2fs: allow compression of files without blocks

Files created by truncate(1) have a size but no blocks, so
they can be allowed to enable compression.

Signed-off-by: Chao Liu <liuchao@coolpad.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>