TCP RACK: don't use an uninitialized variable

When storing the old beta values in rack_swap_beta_values(),
ensure that the newreno_flags field is initialized appropriately
instead of using an uninitialized value.
Since the stored newreno_flags aren't actually used, this fix
should not have any functional change.

Reviewed by: rrs
CID: 1523796
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D48260