vfs: Minor documentation fix

The check_acl inode operation and the IPERM_FLAG_RCU flag are long gone; update
the documentation.
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/porting

@@ -379,10 +379,10 @@ may now be called in rcu-walk mode (nd->flags & LOOKUP_RCU). -ECHILD should be
 returned if the filesystem cannot handle rcu-walk. See
 Documentation/filesystems/vfs.txt for more details.
 
-	permission and check_acl are inode permission checks that are called
-on many or all directory inodes on the way down a path walk (to check for
-exec permission). These must now be rcu-walk aware (flags & IPERM_FLAG_RCU).
-See Documentation/filesystems/vfs.txt for more details.
+	permission is an inode permission check that is called on many or all
+directory inodes on the way down a path walk (to check for exec permission). It
+must now be rcu-walk aware (mask & MAY_NOT_BLOCK).  See
+Documentation/filesystems/vfs.txt for more details.
  
 --
 [mandatory]