fat: rcu-delay unloading nls and freeing sbi

makes ->d_hash() and ->d_compare() safety in RCU mode independent
from vfsmount_lock.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/fat/fat.h

@@ -102,6 +102,7 @@ struct msdos_sb_info {
 	struct hlist_head dir_hashtable[FAT_HASH_SIZE];
 
 	unsigned int dirty;           /* fs state before mount */
+	struct rcu_head rcu;
 };
 
 #define FAT_CACHE_VALID	0	/* special case for valid cache */

fs/fat/inode.c

@@ -548,6 +548,16 @@ static void fat_set_state(struct super_block *sb,
 	brelse(bh);
 }
 
+static void delayed_free(struct rcu_head *p)
+{
+	struct msdos_sb_info *sbi = container_of(p, struct msdos_sb_info, rcu);
+	unload_nls(sbi->nls_disk);
+	unload_nls(sbi->nls_io);
+	if (sbi->options.iocharset != fat_default_iocharset)
+		kfree(sbi->options.iocharset);
+	kfree(sbi);
+}
+
 static void fat_put_super(struct super_block *sb)
 {
 	struct msdos_sb_info *sbi = MSDOS_SB(sb);
@@ -557,14 +567,7 @@ static void fat_put_super(struct super_block *sb)
 	iput(sbi->fsinfo_inode);
 	iput(sbi->fat_inode);
 
-	unload_nls(sbi->nls_disk);
-	unload_nls(sbi->nls_io);
-
-	if (sbi->options.iocharset != fat_default_iocharset)
-		kfree(sbi->options.iocharset);
-
-	sb->s_fs_info = NULL;
-	kfree(sbi);
+	call_rcu(&sbi->rcu, delayed_free);
 }
 
 static struct kmem_cache *fat_inode_cachep;