Fix harmless locking bug in tfp_fastopen_check_cookie().

The keylist lock was not being acquired early enough. The only side
effect of this bug is that the effective add time of a new key could
be slightly later than it would have been otherwise, as seen by a TFO
client.

Reviewed by: tuexen
MFC after: 1 month
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D14046