Introduce reserve_pv_entry() and use it in pmap_pv_demote_pde(). In order

to add PV list locking to pmap_pv_demote_pde(), it is necessary to change
the way that pmap_pv_demote_pde() allocates PV entries.  Specifically,
once pmap_pv_demote_pde() begins modifying the PV lists, it can't allocate
any new PV chunks, because that could require the PV list lock to be
dropped.  So, all necessary PV chunks must be allocated in advance.  To my
surprise, this new approach is a few percent faster than the old one.

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