f2fs: increase the limit for reserve_root

This patch increases the threshold that limits the reserved root space from 0.2%
to 12.5% by using simple shift operation.

Typically Android sets 128MB, but if the storage capacity is 32GB, 0.2% which is
around 64MB becomes too small. Let's relax it.

Cc: stable@vger.kernel.org
Reported-by: Aran Dalton <arda@allwinnertech.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/super.c

@@ -301,10 +301,10 @@ static void f2fs_destroy_casefold_cache(void) { }
 
 static inline void limit_reserve_root(struct f2fs_sb_info *sbi)
 {
-	block_t limit = min((sbi->user_block_count << 1) / 1000,
+	block_t limit = min((sbi->user_block_count >> 3),
 			sbi->user_block_count - sbi->reserved_blocks);
 
-	/* limit is 0.2% */
+	/* limit is 12.5% */
 	if (test_opt(sbi, RESERVE_ROOT) &&
 			F2FS_OPTION(sbi).root_reserved_blocks > limit) {
 		F2FS_OPTION(sbi).root_reserved_blocks = limit;