afs: Don't over-increment the cell usage count when pinning it

AFS cells that are added or set as the workstation cell through /proc are
pinned against removal by setting the AFS_CELL_FL_NO_GC flag on them and
taking a ref.  The ref should be only taken if the flag wasn't already set.

Fix this by making it conditional.

Without this an assertion failure will occur during module removal
indicating that the refcount is too elevated.
Signed-off-by: David Howells <dhowells@redhat.com>

fs/afs/cell.c

@@ -334,7 +334,7 @@ int afs_cell_init(struct afs_net *net, const char *rootcell)
 		return PTR_ERR(new_root);
 	}
 
-	set_bit(AFS_CELL_FL_NO_GC, &new_root->flags);
+	if (!test_and_set_bit(AFS_CELL_FL_NO_GC, &new_root->flags))
 		afs_get_cell(new_root);
 
 	/* install the new cell */

fs/afs/proc.c

@@ -284,7 +284,8 @@ static ssize_t afs_proc_cells_write(struct file *file, const char __user *buf,
 			goto done;
 		}
 
-		set_bit(AFS_CELL_FL_NO_GC, &cell->flags);
+		if (test_and_set_bit(AFS_CELL_FL_NO_GC, &cell->flags))
+			afs_put_cell(net, cell);
 		printk("kAFS: Added new cell '%s'\n", name);
 	} else {
 		goto inval;