optimizations into two distinct pieces. The first piece consists of the
code that should only be performed once per page fault and requires the map
to be locked. The second piece consists of the code that should be
performed each time a pager is called on an object in the shadow chain.
(This second piece expects the map to be unlocked.)
Previously, the entire implementation could be executed multiple times.
Moreover, the second and subsequent executions would occur with the map
unlocked. Usually, the ensuing unsynchronized accesses to the map were
harmless because the map was not changing. Nonetheless, it was possible for
a use-after-free error to occur, where vm_fault() wrote to a freed map
entry. This change corrects that problem.
Reported by: avg
Reviewed by: kib
MFC after: 3 days
Sponsored by: EMC / Isilon Storage Division
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