Implement a variant locking scheme for vm maps: Access to system maps

is now synchronized by a mutex, whereas access to user maps is still
synchronized by a lockmgr()-based lock.  Why?  No single type of lock,
including sx locks, meets the requirements of both types of vm map.
Sometimes we sleep while holding the lock on a user map.  Thus, a
a mutex isn't appropriate.  On the other hand, both lockmgr()-based
and sx locks release Giant when a thread/process blocks during
contention for a lock.  This could lead to a race condition in a legacy
driver (that relies on Giant for synchronization) if it attempts to
kmem_malloc() and fails to immediately obtain the lock.  Fortunately,
we never sleep while holding a system map lock.

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