audit: fix type of sessionid in audit_set_loginuid()

sfr pointed out that with CONFIG_UIDGID_STRICT_TYPE_CHECKS set the audit
tree would not build.  This is because the oldsessionid in
audit_set_loginuid() was accidentally being declared as a kuid_t.  This
patch fixes that declaration mistake.

Example of problem:
kernel/auditsc.c: In function 'audit_set_loginuid':
kernel/auditsc.c:2003:15: error: incompatible types when assigning to
type 'kuid_t' from type 'int'
  oldsessionid = audit_get_sessionid(current);
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Eric Paris <eparis@redhat.com>

kernel/auditsc.c

@@ -1995,8 +1995,8 @@ static void audit_log_set_loginuid(kuid_t koldloginuid, kuid_t kloginuid,
 int audit_set_loginuid(kuid_t loginuid)
 {
 	struct task_struct *task = current;
-	unsigned int sessionid = -1;
-	kuid_t oldloginuid, oldsessionid;
+	unsigned int oldsessionid, sessionid = (unsigned int)-1;
+	kuid_t oldloginuid;
 	int rc;
 
 	oldloginuid = audit_get_loginuid(current);