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