x86/mm: Refactor modify_xen_mappings to have one exit path

We will soon need to perform clean-ups before returning.

No functional change.

This is part of XSA-345.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Hongyan Xia <hongyxia@amazon.com>
Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -5564,6 +5564,7 @@ int modify_xen_mappings(unsigned long s,
     l1_pgentry_t *pl1e;
     unsigned int  i;
     unsigned long v = s;
+    int rc = -ENOMEM;
 
     /* Set of valid PTE bits which may be altered. */
 #define FLAGS_MASK (_PAGE_NX|_PAGE_RW|_PAGE_PRESENT)
@@ -5605,7 +5606,8 @@ int modify_xen_mappings(unsigned long s,
             /* PAGE1GB: shatter the superpage and fall through. */
             pl2e = alloc_xen_pagetable();
             if ( !pl2e )
-                return -ENOMEM;
+                goto out;
+
             for ( i = 0; i < L2_PAGETABLE_ENTRIES; i++ )
                 l2e_write(pl2e + i,
                           l2e_from_pfn(l3e_get_pfn(*pl3e) +
@@ -5660,7 +5662,8 @@ int modify_xen_mappings(unsigned long s,
                 /* PSE: shatter the superpage and try again. */
                 pl1e = alloc_xen_pagetable();
                 if ( !pl1e )
-                    return -ENOMEM;
+                    goto out;
+
                 for ( i = 0; i < L1_PAGETABLE_ENTRIES; i++ )
                     l1e_write(&pl1e[i],
                               l1e_from_pfn(l2e_get_pfn(*pl2e) + i,
@@ -5789,7 +5792,10 @@ int modify_xen_mappings(unsigned long s,
     flush_area(NULL, FLUSH_TLB_GLOBAL);
 
 #undef FLAGS_MASK
-    return 0;
+    rc = 0;
+
+ out:
+    return rc;
 }
 
 #undef flush_area