x86/xen: add reschedule point when mapping foreign GFNs

Mapping a large range of foreign GFNs can take a long time, add a
reschedule point after each batch of 16 GFNs.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index 9c479fe..ac161db 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -2888,6 +2888,7 @@
 		addr += range;
 		if (err_ptr)
 			err_ptr += batch;
+		cond_resched();
 	}
 out: