f2fs: fix to update {m,c}time correctly when truncating larger

f2fs: fix to update {m,c}time correctly when truncating larger

This patch fixes to update ctime and atime correctly when truncating
larger in ->setattr.

The bug is reported by xfstest generic/313 as below:

generic/313 2s ... - output mismatch (see ./results/generic/313.out.bad)
    --- tests/generic/313.out   2015-08-04 15:28:53.430798882 +0800
    +++ results/generic/313.out.bad   2015-09-28 17:04:27.294278016 +0800
    @@ -1,2 +1,4 @@
     QA output created by 313
     Silence is golden
    +ctime not updated after truncate up
    +mtime not updated after truncate up
    ...
    (Run 'diff -u tests/generic/313.out tests/generic/313.out.bad'  to see the entire diff)
Ran: generic/313
Failures: generic/313
Failed 1 of 1 tests

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>