SUNRPC: Move TCP state flags into xprtsock.c

Move "XPRT_LAST_FRAG" and friends from xprt.h into xprtsock.c, and rename
them to use the naming scheme in use in xprtsock.c.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>