Update nfsrv_getsocksndseq() for changes in TCP internals since FreeBSD 6.x:
Update nfsrv_getsocksndseq() for changes in TCP internals since FreeBSD 6.x:

- so_pcb is now guaranteed to be non-NULL and valid if a valid socket
  reference is held.

- Need to check INP_TIMEWAIT and INP_DROPPED before assuming inp_ppcb is a
  tcpcb, as it might be a tcptw or NULL otherwise.

- tp can never be NULL by the end of the function, so only check
  TCPS_ESTABLISHED before extracting tcpcb fields.

The NFS server arguably incorporates too many assumptions about TCP
internals, but fixing that is left for nother day.

MFC after:		1 week
Reviewed by:		bz
Reviewed and tested by:	rmacklem
Sponsored by:		Juniper Networks

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