xen: speed up grant-table reclaim

In the Linux kernel, the following vulnerability has been resolved:

xen: speed up grant-table reclaim

When a grant entry is still in use by the remote domain, Linux must put

it on a deferred list. Normally, this list is very short, because

the PV network and block protocols expect the backend to unmap the grant

first. However, Qubes OS's GUI protocol is subject to the constraints

of the X Window System, and as such winds up with the frontend unmapping

the window first. As a result, the list can grow very large, resulting

in a massive memory leak and eventual VM freeze.

To partially solve this problem, make the number of entries that the VM

will attempt to free at each iteration tunable. The default is still

10, but it can be overridden via a module parameter.

This is Cc: stable because (when combined with appropriate userspace

changes) it fixes a severe performance and stability problem for Qubes

OS users.