gnttab: correct locking on transitive grant copy error path

gnttab: correct locking on transitive grant copy error path

While the comment next to the lock dropping in preparation of
recursively calling acquire_grant_for_copy() mistakenly talks about the
rd == td case (excluded a few lines further up), the same concerns apply
to the calling of release_grant_for_copy() on a subsequent error path.

This is CVE-2022-33748 / XSA-411.

Fixes: ad48fb963dbf ("gnttab: fix transitive grant handling")
Signed-off-by: Jan Beulich <jbeulich@suse.com>