hpfs: make freeing sbi and codetables rcu-delayed

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/hpfs/hpfs_fn.h

@@ -80,6 +80,7 @@ struct hpfs_sb_info {
 	unsigned sb_c_bitmap;		/* current bitmap */
 	unsigned sb_max_fwd_alloc;	/* max forwad allocation */
 	int sb_timeshift;
+	struct rcu_head rcu;
 };
 
 /* Four 512-byte buffers and the 2k block obtained by concatenating them */