f2fs: set the default compress_level on ioctl

Otherwise, we'll get a broken inode.

 # touch $FILE
 # f2fs_io setflags compression $FILE
 # f2fs_io set_coption 2 8 $FILE

[  112.227612] F2FS-fs (dm-51): sanity_check_compress_inode: inode (ino=8d3fe) has unsupported compress level: 0, run fsck to fix

Cc: stable@vger.kernel.org
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>