f2fs: fix compile warning

This patch fixes below compile warning reported by LKP
(kernel test robot)

cppcheck warnings: (new ones prefixed by >>)

>> fs/f2fs/file.c:761:9: warning: Identical condition 'err', second condition is always false [identicalConditionAfterEarlyExit]
    return err;
           ^
   fs/f2fs/file.c:753:6: note: first condition
    if (err)
        ^
   fs/f2fs/file.c:761:9: note: second condition
    return err;
Reported-by: Nkernel test robot <lkp@intel.com>
Signed-off-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/file.c

@@ -753,11 +753,14 @@ int f2fs_truncate_blocks(struct inode *inode, u64 from, bool lock)
 		return err;
 
 #ifdef CONFIG_F2FS_FS_COMPRESSION
-	if (from != free_from)
+	if (from != free_from) {
 		err = f2fs_truncate_partial_cluster(inode, from, lock);
+		if (err)
+			return err;
+	}
 #endif
 
-	return err;
+	return 0;
 }
 
 int f2fs_truncate(struct inode *inode)