Revert 22186:7167d6dd5c7c "x86: Retry do_mmu_update() a few times"

It does not work reliably for a couple of reasons:
(1) page_lock() fails if a page is !PGT_validated, and a page can
remain in that state for unbounded time.
(2) in the kernel-side race that motivated this patch, pgd_pin() can
lose to vmalloc_sync_all() -- pgd_pin() can try to chaneg a pmd page's
type to l2_pagetable while
vmalloc_sync_all()->set_pmd()->do_mmu_update() has it temporarily
pinned as writable. This is hard to fix on the Xen side.

Hence I give up on this approach, revert the patch, and settle for
kernel-side patching only.

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>

xen/arch/x86/mm.c

@@ -3414,7 +3414,6 @@ int do_mmu_update(
         case MMU_NORMAL_PT_UPDATE:
         case MMU_PT_UPDATE_PRESERVE_AD:
         {
-            unsigned int retries = 0;
             p2m_type_t p2mt;
 
             rc = xsm_mmu_normal_update(d, pg_owner, req.val);
@@ -3446,7 +3445,6 @@ int do_mmu_update(
                           (unsigned long)(req.ptr & ~PAGE_MASK));
             page = mfn_to_page(mfn);
 
-        retry:
             if ( page_lock(page) )
             {
                 switch ( page->u.inuse.type_info & PGT_type_mask )
@@ -3603,11 +3601,6 @@ int do_mmu_update(
                     v, va, req.val, _mfn(mfn));
                 put_page_type(page);
             }
-            else if ( retries++ < 5 )
-            {
-                /* Page type can be in flux, so we retry a few times. */
-                goto retry;
-            }
 
             unmap_domain_page_with_cache(va, &mapcache);
             put_page(page);