integrity: support new struct public_key_signature encoding field

On systems with IMA-appraisal enabled with a policy requiring file
signatures, the "good" signature values are stored on the filesystem as
extended attributes (security.ima).  Signature verification failure
would normally be limited to just a particular file (eg. executable),
but during boot signature verification failure could result in a system
hang.

Defining and requiring a new public_key_signature field requires all
callers of asymmetric signature verification to be updated to reflect
the change.  This patch updates the integrity asymmetric_verify()
caller.

Fixes: 82f94f24

 ("KEYS: Provide software public key query function [ver #2]")
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
Cc: David Howells <dhowells@redhat.com>
Acked-by: Denis Kenzior <denkenz@gmail.com>