gve: Clear can_flip flag after flip the buffer.

Otherwise, we may flip the buffer which we shouldn't if the descriptor
is dropped.

BUG=b/171000160

Change-Id: I3e9c376d24203d7cd701a9f1e93e6021ea49db51
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/2141
Reviewed-by: Roy Yang <royyang@google.com>
Reviewed-by: Yangchun Fu <yangchun@google.com>
(cherry picked from commit c63c54a7d9efc61d38c5f4e201ebf81f34cb08e2)
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/7621
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>