Make preparations for resurrecting shared/read locks on vm maps:

mac_proc_vm_revoke_recurse() requests a read lock on the vm map at the start
but does not handle failure by vm_map_lock_upgrade() when it seeks to modify
the vm map.  At present, this works because all lock request on a vm map are
implemented as exclusive locks.  Thus, vm_map_lock_upgrade() is a no-op that
always reports success.  However, that is about to change, and
proc_vm_revoke_recurse() will require substantial modifications to handle
vm_map_lock_upgrade() failures.  For the time being, I am changing
mac_proc_vm_revoke_recurse() to request a write lock on the vm map at the
start.

Approved by:	rwatson
MFC after:	3 months

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