selinux: Add accessor functions for inode->i_security

selinux: Add accessor functions for inode->i_security

Add functions dentry_security and inode_security for accessing
inode->i_security.  These functions initially don't do much, but they
will later be used to revalidate the security labels when necessary.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <pmoore@redhat.com>
Orabug: 25684456

(backport upstream commit 83da53c5a34564a0a63b26f84293c6e2a639e1e4)

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Acked-by: James Morris <james.l.morris@oracle.com>
Reviewed-by: James Morris <james.l.morris@oracle.com>
 conflict fix
    security/selinux/hooks.c