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>