x86/mm/p2m: Fix locking discipline around p2m lookups.

x86/mm/p2m: Fix locking discipline around p2m lookups.

All gfn_to_mfn* functions except _query() might take the p2m lock,
so can't be called with a p2m, shadow, hap or log_dirty lock held.
The remaining offender is the memory sharing code, which calls
_unshare() from inside the pagetable walker!  Fixing that is too big
for a cleanup patch like this one.

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