TCP BBR: fix getsockopt() for TCP_BBR_USEDEL_RATE

Actually implement the IPPROTO_TCP-level socket option
TCP_BBR_USEDEL_RATE.

Reviewed by: rrs
CID: 1523813
CID: 1523814
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D48261