When not holding the PoD lock across the entire region covering P2M
update and stats update, the entry count should indicate too large a
value in preference to a too small one, to avoid functions bailing early
when they find the count is zero. Hence increments should happen ahead
of P2M updates, while decrements should happen only after. Deal with the
one place where this hasn't been the case yet.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
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v2: Add comments.
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While it might be possible to hold the PoD lock over the entire
operation, I didn't want to chance introducing a lock order violation on
a perhaps rarely taken code path.