lsm: cleanup the size counters in security_getselfattr()

Zero out all of the size counters in the -E2BIG case (buffer too
small) to help make the current code a bit more robust in the face of
future code changes.

Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Reviewed-by: Mickaël Salaün <mic@digikod.net>
Signed-off-by: Paul Moore <paul@paul-moore.com>