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>

fs/f2fs/super.c

@@ -213,7 +213,8 @@ void f2fs_msg(struct super_block *sb, const char *level, const char *fmt, ...)
 
 static inline void limit_reserve_root(struct f2fs_sb_info *sbi)
 {
-	block_t limit = (sbi->user_block_count << 1) / 1000;
+	block_t limit = min((sbi->user_block_count << 1) / 1000,
+			sbi->user_block_count - sbi->reserved_blocks);
 
 	/* limit is 0.2% */
 	if (test_opt(sbi, RESERVE_ROOT) &&