f2fs: fix to alloc_mode changed after remount on a small volume device

The commit 84b89e5d ("f2fs: add auto tuning for small devices") add
tuning for small volume device, now support to tune alloce_mode to 'reuse'
if it's small size. But the alloc_mode will change to 'default' when do
remount on this small size dievce. This patch fo fix alloc_mode changed
when do remount for a small volume device.

Signed-off-by: Yuwei Guan <Yuwei.Guan@zeekrlife.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>