f2fs: remove set but not used variable 'err'

Fixes gcc '-Wunused-but-set-variable' warning:

fs/f2fs/data.c: In function 'f2fs_dio_submit_bio':
fs/f2fs/data.c:2585:6: warning:
 variable 'err' set but not used [-Wunused-but-set-variable]

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