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
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D48261

(cherry picked from commit b47dcb4b1ff16e6af2e07798f6fe06580b4eddf0)