Switch to using the inpcb MAC label instead of socket MAC label when
labeling new mbufs created from sockets/inpcbs in IPv4. This helps avoid
the need for socket layer locking in the lower level network paths
where inpcb locks are already frequently held where needed. In
particular:

• Use the inpcb for label instead of socket in raw_append().
• Use the inpcb for label instead of socket in tcp_output().
• Use the inpcb for label instead of socket in tcp_respond().
• Use the inpcb for label instead of socket in tcp_twrespond().
• Use the inpcb for label instead of socket in syncache_respond().

While here, modify tcp_respond() to avoid assigning NULL to a stack
variable and centralize assertions about the inpcb when inp is
assigned.

Obtained from: TrustedBSD Project
Sponsored by: DARPA, McAfee Research