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

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>