dtrace: improve siftr probe

Improve consistency of the field names with tcpsinfo_t:

• Use mss instead of max_seg_size.
• Use lport and rport instead of tcp_localport and tcp_foreignport.

Use t_flags instead of flags to improve consistency with t_flags2.

Add laddr and raddr, since the addresses were missing when compared
to the output of siftr.

Reviewed by: cc
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D40834