integrity: Add an integrity directory in securityfs

We want to add additional evm control nodes, and it'd be preferable not
to clutter up the securityfs root directory any further. Create a new
integrity directory, move the ima directory into it, create an evm
directory for the evm attribute and add compatibility symlinks.

Signed-off-by: Matthew Garrett <mjg59@google.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>