x86/mm: Introduce get_page_from_gfn().

x86/mm: Introduce get_page_from_gfn().

This new function does a p2m lookup under the read lock, falling back
to the write lock only if it needs to make a change.  If the GFN is
backed by RAM, it takes a refcount on the underlying page.

The following patches will convert many paths that currently use
get_gfn/put_gfn to use the new interface.  That will avoid serializing
p2m accesses in the common case where no updates are needed (i.e. no
page-sharing, VM paging or other p2m trickery).

Signed-off-by: Tim Deegan <tim@xen.org>