Apparmor: mediated_filesystem() should use dentry->d_sb not inode->i_sb

mediated_filesystem() should use dentry->d_sb not dentry->d_inode->i_sb and
should avoid file_inode() also since it is really dealing with the path.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>