Backport of 166d38632316 ("xen/gntdev: Ignore failure to unmap INVALID_GRANT_HANDLE")

[Demi Marie Obenour]

This series backports upstream commit 166d3863231667c4f64dee72b77d1102cdfad11f
to all supported stable kernel trees.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab