x86/mm/p2m: merge gfn_to_mfn_unshare with other gfn_to_mfn paths.

x86/mm/p2m: merge gfn_to_mfn_unshare with other gfn_to_mfn paths.

gfn_to_mfn_unshare() had its own function despite all other lookup types
being handled in one place. Merge it into _gfn_to_mfn_type(), so that it
gets the benefit of broken-page protection, for example, and tidy its
interfaces up to fit.

The unsharing code still has a lot of bugs, e.g.
 - failure to alloc for unshare on a foreign lookup still BUG()s,
 - at least one race condition in unshare-and-retry
 - p2m_* lookup types should probably be flags, not enum
but it's cleaner and will make later p2m cleanups easier.

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