x86/mm: simplify log-dirty page allocation.

x86/mm: simplify log-dirty page allocation.

Now that the log-dirty code is covered by the same lock as shadow and
hap activity, we no longer need to avoid doing allocs and frees with
the lock held.  Simplify the code accordingly.

Signed-off-by: Tim Deegan <Tim.Deegan@citrix.com>