afs: prevent double cell registration

kafs doesn't check if the cell already exists - so if you do an echo "add
newcell.org 1.2.3.4" >/proc/fs/afs/cells it will try to create this cell
again.  kobject will also complain about a double registration.  To prevent
such problems, return -EEXIST in that case.
Signed-off-by: Sven Schnelle <svens@stackframe.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/afs/cell.c

@@ -127,14 +127,20 @@ struct afs_cell *afs_cell_create(const char *name, char *vllist)
 
 	_enter("%s,%s", name, vllist);
 
+	down_write(&afs_cells_sem);
+	read_lock(&afs_cells_lock);
+	list_for_each_entry(cell, &afs_cells, link) {
+		if (strcasecmp(cell->name, name) == 0)
+			goto duplicate_name;
+	}
+	read_unlock(&afs_cells_lock);
+
 	cell = afs_cell_alloc(name, vllist);
 	if (IS_ERR(cell)) {
 		_leave(" = %ld", PTR_ERR(cell));
 		return cell;
 	}
 
-	down_write(&afs_cells_sem);
-
 	/* add a proc directory for this cell */
 	ret = afs_proc_cell_setup(cell);
 	if (ret < 0)
@@ -167,6 +173,11 @@ error:
 	kfree(cell);
 	_leave(" = %d", ret);
 	return ERR_PTR(ret);
+
+duplicate_name:
+	read_unlock(&afs_cells_lock);
+	up_write(&afs_cells_sem);
+	return ERR_PTR(-EEXIST);
 }
 
 /*