Fix seqno mismatch in fly()

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().