MFC r341335:

Limit option_len for the TCP_CCALGOOPT.

Limiting the length to 2048 bytes seems to be acceptable, since
the values used right now are using 8 bytes.
This issue was found by using syzkaller.

Reviewed by: glebius, bz, rrs
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D18366