auditsc: Use kzalloc instead of kmalloc+memset.

  In function audit_alloc_context(), use kzalloc, instead of kmalloc+memset. Patch also renames audit_zero_context() to
audit_set_context(), to represent it's inner workings properly.

Signed-off-by: Rakib Mullick <rakib.mullick@gmail.com>
Signed-off-by: Eric Paris <eparis@redhat.com>