x86/mm: Take the p2m lock even in shadow mode.

x86/mm: Take the p2m lock even in shadow mode.

The reworking of p2m lookups to use get_gfn()/put_gfn() left the
shadow code not taking the p2m lock, even in cases where the p2m would
be updated (i.e. PoD).

In many cases, shadow code doesn't need the exclusion that
get_gfn()/put_gfn() provides, as it has its own interlocks against p2m
updates, but this is taking things too far, and can lead to crashes in
the PoD code.

Now that most shadow-code p2m lookups are done with explicitly
unlocked accessors, or with the get_page_from_gfn() accessor, which is
often lock-free, we can just turn this locking on.

The remaining locked lookups are in sh_page_fault() (in a path that's
almost always already serializing on the paging lock), and in
emulate_map_dest() (which can probably be updated to use
get_page_from_gfn()).  They're not addressed here but may be in a
follow-up patch.

Signed-off-by: Tim Deegan <tim@xen.org>
Acked-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>