cifs: rcu-delay unload_nls() and freeing sbi

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

fs/cifs/cifs_fs_sb.h

@@ -65,5 +65,6 @@ struct cifs_sb_info {
 	char   *mountdata; /* options received at mount time or via DFS refs */
 	struct backing_dev_info bdi;
 	struct delayed_work prune_tlinks;
+	struct rcu_head rcu;
 };
 #endif				/* _CIFS_FS_SB_H */

fs/cifs/connect.c

@@ -3755,6 +3755,13 @@ CIFSTCon(const unsigned int xid, struct cifs_ses *ses,
 	return rc;
 }
 
+static void delayed_free(struct rcu_head *p)
+{
+	struct cifs_sb_info *sbi = container_of(p, struct cifs_sb_info, rcu);
+	unload_nls(sbi->local_nls);
+	kfree(sbi);
+}
+
 void
 cifs_umount(struct cifs_sb_info *cifs_sb)
 {
@@ -3779,8 +3786,7 @@ cifs_umount(struct cifs_sb_info *cifs_sb)
 
 	bdi_destroy(&cifs_sb->bdi);
 	kfree(cifs_sb->mountdata);
-	unload_nls(cifs_sb->local_nls);
-	kfree(cifs_sb);
+	call_rcu(&cifs_sb->rcu, delayed_free);
 }
 
 int