intel: Add an interface to limit vma caching

intel: Add an interface to limit vma caching

There is a per-process limit on the number of vma that the process can
keep open, so we cannot keep an unlimited cache of unused vma's (besides
keeping track of all those vma in the kernel adds considerable overhead).
However, in order to work around inefficiencies in the kernel it is
beneficial to reuse the vma, so keep a MRU cache of vma.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>