xen/p2m: Reuse existing P2M leafs if they are filled with 1:1 PFNs or INVALID.

If P2M leaf is completly packed with INVALID_P2M_ENTRY or with
1:1 PFNs (so IDENTITY_FRAME type PFNs), we can swap the P2M leaf
with either a p2m_missing or p2m_identity respectively. The old
page (which was created via extend_brk or was grafted on from the
mfn_list) can be re-used for setting new PFNs.

This also means we can remove git commit:
5bc6f988
xen/p2m: Reserve 8MB of _brk space for P2M leafs when populating back
which tried to fix this.

and make the amount that is required to be reserved much smaller.

CC: stable@vger.kernel.org # for 3.5 only.
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>