kernel/x86_64: Fix descriptor index off-by-one in assert.

This was apparently copied from GlobalDescriptorTable::SetTSS() which
needs two table entries and therefore checks for index + 1. For the
SetUserTLS case this isn't needed and would cause aborts when reaching
the maximum CPU count (64 currently).

Change-Id: I27bd777fedadbd3740ac8c791199ec9300b06327
Reviewed-on: https://review.haiku-os.org/809
Reviewed-by: waddlesplash <[email protected]>

src/system/kernel/arch/x86/64/descriptors.cpp

@@ -280,7 +280,7 @@ unsigned
 GlobalDescriptorTable::SetUserTLS(unsigned cpu, uintptr_t base, size_t limit)
 {
 	auto index = kFirstTSS + cpu * 3 + 2;
-	ASSERT(index + 1 < kDescriptorCount);
+	ASSERT(index < kDescriptorCount);
 	UserTLSDescriptor desc(base, limit);
 	fTable[index] = desc.GetDescriptor();
 	return index;