grant_table: Race conditions with maptrack free list handling

ISSUE DESCRIPTION

The grant table code in Xen has a bespoke semi-lockfree allocator for recording grant mappings (“maptrack” entries). This allocator has a race which allows the free list to be corrupted.
Specifically: the code for removing an entry from the free list, prior to use, assumes (without locking) that if inspecting head item shows that it is not the tail, it will continue to not be the tail of the list if it is later found to be still the head and removed with cmpxchg. But the entry might have been removed and replaced, with the result that it might be the tail by then. (The invariants for the semi-lockfree data structure were never formally documented.)
Additionally, a stolen entry is put on the free list with an incorrect link field, which will very likely corrupt the list.

IMPACT

A malicious guest administrator can crash the host, and can probably escalate their privilege to that of the host.

VULNERABLE SYSTEMS

Xen 4.6 and later are vulnerable.
Xen 4.5 and earlier are not vulnerable.