Fix double-free on error condition in GnttabPermitForeignAccess()

Fix double-free on error condition in GnttabPermitForeignAccess()

XENIFACE_GNTTAB_CONTEXT associated with the request was incorrectly freed
by GnttabPermitForeignAccess() when a failure occured. The context is also
freed by the parent function, IoctlGnttabPermitForeignAccess(), which led
to a double-free and kernel heap corruption.

GnttabStopSharing() as the final step in the failure path takes care
of cleaning up the contents of the (possibly partially initialized) context.

Signed-off-by: Rafał Wojdyła <omeg@invisiblethingslab.com>
Reviewed-by: Owen Smith <owen.smith@cloud.com>