xen/p2m: Don't call get_balloon_scratch_page() twice, keep interrupts disabled for multicalls

m2p_remove_override() calls get_balloon_scratch_page() in
MULTI_update_va_mapping() even though it already has pointer to this page from
the earlier call (in scratch_page). This second call doesn't have a matching
put_balloon_scratch_page() thus not restoring preempt count back. (Also, there
is no put_balloon_scratch_page() in the error path.)

In addition, the second multicall uses __xen_mc_entry() which does not disable
interrupts. Rearrange xen_mc_* calls to keep interrupts off while performing
multicalls.

This commit fixes a regression introduced by:

commit ee0726407feaf504dff304fb603652fb2d778b42
Author: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Date:   Tue Jul 23 17:23:54 2013 +0000

    xen/m2p: use GNTTABOP_unmap_and_replace to reinstate the original mapping

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>

arch/x86/xen/p2m.c

@@ -990,10 +990,13 @@ int m2p_remove_override(struct page *page,
 				printk(KERN_WARNING "m2p_remove_override: "
 						"pfn %lx mfn %lx, failed to modify kernel mappings",
 						pfn, mfn);
+				put_balloon_scratch_page();
 				return -1;
 			}
 
-			mcs = xen_mc_entry(
+			xen_mc_batch();
+
+			mcs = __xen_mc_entry(
 					sizeof(struct gnttab_unmap_and_replace));
 			unmap_op = mcs.args;
 			unmap_op->host_addr = kmap_op->host_addr;
@@ -1003,12 +1006,11 @@ int m2p_remove_override(struct page *page,
 			MULTI_grant_table_op(mcs.mc,
 					GNTTABOP_unmap_and_replace, unmap_op, 1);
 
-			xen_mc_issue(PARAVIRT_LAZY_MMU);
-
 			mcs = __xen_mc_entry(0);
 			MULTI_update_va_mapping(mcs.mc, scratch_page_address,
-					pfn_pte(page_to_pfn(get_balloon_scratch_page()),
+					pfn_pte(page_to_pfn(scratch_page),
 					PAGE_KERNEL_RO), 0);
+
 			xen_mc_issue(PARAVIRT_LAZY_MMU);
 
 			kmap_op->host_addr = 0;