f2fs: allow set compression option of files without blocks

Files created by truncate have a size but no blocks, so
they can be allowed to set compression option.

Fixes: e1e8debe ("f2fs: add F2FS_IOC_SET_COMPRESS_OPTION ioctl")
Signed-off-by: Yangtao Li <frank.li@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>