exfat: support dynamic allocate bh for exfat_entry_set_cache

In special cases, a file or a directory may occupied more than 19
directory entries, pre-allocating 3 bh is not enough. Such as
  - Support vendor secondary directory entry in the future.
  - Since file directory entry is damaged, the SecondaryCount
    field is bigger than 18.

So this commit supports dynamic allocation of bh.

Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Reviewed-by: Andy Wu <Andy.Wu@sony.com>
Reviewed-by: Aoyama Wataru <wataru.aoyama@sony.com>
Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>

fs/exfat/dir.c

@@ -615,6 +615,10 @@ int exfat_free_dentry_set(struct exfat_entry_set_cache *es, int sync)
 			bforget(es->bh[i]);
 		else
 			brelse(es->bh[i]);
+
+	if (IS_DYNAMIC_ES(es))
+		kfree(es->bh);
+
 	kfree(es);
 	return err;
 }
@@ -847,6 +851,7 @@ struct exfat_entry_set_cache *exfat_get_dentry_set(struct super_block *sb,
 	/* byte offset in sector */
 	off = EXFAT_BLK_OFFSET(byte_offset, sb);
 	es->start_off = off;
+	es->bh = es->__bh;
 
 	/* sector offset in cluster */
 	sec = EXFAT_B_TO_BLK(byte_offset, sb);
@@ -866,6 +871,16 @@ struct exfat_entry_set_cache *exfat_get_dentry_set(struct super_block *sb,
 	es->num_entries = num_entries;
 
 	num_bh = EXFAT_B_TO_BLK_ROUND_UP(off + num_entries * DENTRY_SIZE, sb);
+	if (num_bh > ARRAY_SIZE(es->__bh)) {
+		es->bh = kmalloc_array(num_bh, sizeof(*es->bh), GFP_KERNEL);
+		if (!es->bh) {
+			brelse(bh);
+			kfree(es);
+			return NULL;
+		}
+		es->bh[0] = bh;
+	}
+
 	for (i = 1; i < num_bh; i++) {
 		/* get the next sector */
 		if (exfat_is_last_sector_in_cluster(sbi, sec)) {

fs/exfat/exfat_fs.h

@@ -185,11 +185,14 @@ struct exfat_entry_set_cache {
 	struct super_block *sb;
 	unsigned int start_off;
 	int num_bh;
-	struct buffer_head *bh[DIR_CACHE_SIZE];
+	struct buffer_head *__bh[DIR_CACHE_SIZE];
+	struct buffer_head **bh;
 	unsigned int num_entries;
 	bool modified;
 };
 
+#define IS_DYNAMIC_ES(es)	((es)->__bh != (es)->bh)
+
 struct exfat_dir_entry {
 	struct exfat_chain dir;
 	int entry;