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>
diff --git a/fs/fat/fat.h b/fs/fat/fat.h
index 4241e6f..7c31f4b 100644
--- a/fs/fat/fat.h
+++ b/fs/fat/fat.h
@@ -102,6 +102,7 @@
 	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 */