x86/xen: only warn once if bad MFNs are found during setup

In xen_add_extra_mem(), if the WARN() checks for bad MFNs trigger it is
likely that they will trigger at lot, spamming the log.

Use WARN_ONCE() instead.
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Tested-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/xen/setup.c

@@ -89,9 +89,9 @@ static void __init xen_add_extra_mem(u64 start, u64 size)
 	for (pfn = PFN_DOWN(start); pfn < xen_max_p2m_pfn; pfn++) {
 		unsigned long mfn = pfn_to_mfn(pfn);
 
-		if (WARN(mfn == pfn, "Trying to over-write 1-1 mapping (pfn: %lx)\n", pfn))
+		if (WARN_ONCE(mfn == pfn, "Trying to over-write 1-1 mapping (pfn: %lx)\n", pfn))
 			continue;
-		WARN(mfn != INVALID_P2M_ENTRY, "Trying to remove %lx which has %lx mfn!\n",
+		WARN_ONCE(mfn != INVALID_P2M_ENTRY, "Trying to remove %lx which has %lx mfn!\n",
 			  pfn, mfn);
 
 		__set_phys_to_machine(pfn, INVALID_P2M_ENTRY);