f2fs: fix to return success when trimming meta area

generic/251
    --- tests/generic/251.out	2016-05-03 20:20:11.381899000 +0800
     QA output created by 251
     Running the test: done.
    +fstrim: /mnt/scratch_f2fs: FITRIM ioctl failed: Invalid argument
    +fstrim: /mnt/scratch_f2fs: FITRIM ioctl failed: Invalid argument
    +fstrim: /mnt/scratch_f2fs: FITRIM ioctl failed: Invalid argument
    +fstrim: /mnt/scratch_f2fs: FITRIM ioctl failed: Invalid argument
    +fstrim: /mnt/scratch_f2fs: FITRIM ioctl failed: Invalid argument
    ...
Ran: generic/251
Failures: generic/251

The reason is coverage of fstrim locates in meta area, previously we
just return -EINVAL for such case, making generic/251 failed, to fix
this problem, let's relieve restriction to return success with no
block discarded.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -2631,8 +2631,8 @@ int f2fs_trim_fs(struct f2fs_sb_info *sbi, struct fstrim_range *range)
 	if (start >= MAX_BLKADDR(sbi) || range->len < sbi->blocksize)
 		return -EINVAL;
 
-	if (end <= MAIN_BLKADDR(sbi))
-		return -EINVAL;
+	if (end < MAIN_BLKADDR(sbi))
+		goto out;
 
 	if (is_sbi_flag_set(sbi, SBI_NEED_FSCK)) {
 		f2fs_msg(sbi->sb, KERN_WARNING,