x86/apic: Simplify X2APIC ID validation

Currently, x2apic_max_apicid==0 means that there is no max APIC id limit.
But, this means that 0 needs to be special-cased.

Designate UINT_MAX to mean unlimited so that a plain old less than or equal
compare works and there is no special-casing.  Replace the 0 initialization
with UINT_MAX.

[ dhansen: muck with changelog ]

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Michael Kelley <mikelley@microsoft.com>
Tested-by: Sohil Mehta <sohil.mehta@intel.com>
Tested-by: Juergen Gross <jgross@suse.com> # Xen PV (dom0 and unpriv. guest)