f2fs: Fix root reserved on remount

On a remount, you can currently set root reserved if it was not
previously set. This can cause an underflow if reserved has been set to
a very high value, since then root reserved + current reserved could be
greater than user_block_count. inc_valid_block_count later subtracts out
these values from user_block_count, causing an underflow.

Signed-off-by: Daniel Rosenberg <drosen@google.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>