f2fs: fix incorrect range->len in f2fs_trim_fs()

generic/260 reported below error:

     [+] Default length with start set (should succeed)
     [+] Length beyond the end of fs (should succeed)
     [+] Length beyond the end of fs with start set (should succeed)
    +./tests/generic/260: line 94: [: 18446744073709551615: integer expression expected
    +./tests/generic/260: line 104: [: 18446744073709551615: integer expression expected
     Test done
    ...

In f2fs_trim_fs(), if there is no discard being trimmed, we need to correct
range->len before return.

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