tcp: refactor register_tcp_functions_as_names()

Refactor register_tcp_functions_as_names() such that either all or
no (in error cases) registrations happen atomically (while holding
the tcp_function_lock write lock). Also ensure that the TCP function
block is not already registered.
This avoids situations, where some registrations were performed and
then they were removed without holding a lock in between or checking
ref counts.

Reviewed by: cc
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D45947