apparmor: add special .null file used to "close" fds at exec

Borrow the special null device file from selinux to "close" fds that
don't have sufficient permissions at exec time.

Signed-off-by: John Johansen <john.johansen@canonical.com>

security/apparmor/include/apparmorfs.h

@@ -15,6 +15,8 @@
 #ifndef __AA_APPARMORFS_H
 #define __AA_APPARMORFS_H
 
+extern struct path aa_null;
+
 enum aa_fs_type {
 	AA_FS_TYPE_BOOLEAN,
 	AA_FS_TYPE_STRING,