Fix seqno mismatch in fly()
fly() reads the carrier, then passes it to pln_dropoff(), which writes
it back. fly() also calls pln_oneway_to_carrier_ok(), which updates
the carrier when its plane summary information is incorrect.
The old code called it between reading the carrier and passing it to
pln_dropoff(). This made pln_dropoff() wipe out the plane summary
update, and triggered a seqno mismatch oops. Broken by introduction
of pln_oneway_to_carrier_ok() in commit
1127762c, v4.2.17.
Fix by reading the carrier right before passing it to pln_dropoff().
(cherry picked from commit
42d9475d894f6779dab17655cd5082b66b9acac9)