tcp rack, bbr: improve handling of soft errors

Do not report an error, if it is stored as a soft error. This avoids,
for example, the dropping of TCP connections using an interface,
while enabling or disabling LRO on that interface.

Reviewed by: cc
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D46427