x86: sparse warnings in pageattr.c

Adjust the definition of lookup_address to take an unsigned long
level argument.  Adjust callers in xen/mmu.c that pass in a
dummy variable.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>