f2fs: allow to clear F2FS_COMPR_FL flag

If regular inode has no compressed cluster, allow using 'chattr -c'
to remove its compress flag, recovering it to a non-compressed file.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>