f2fs: fix to account compressed inode correctly

stat_inc_compr_inode() needs to check FI_COMPRESSED_FILE flag, so
in f2fs_disable_compressed_file(), we should call stat_dec_compr_inode()
before clearing FI_COMPRESSED_FILE flag.

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