MFC r306443:

Fix an issue with accept_filter introduced with r261242:

As a side effect of r261242 when using accept_filter the
first call to soisconnected() is done earlier in tcp_input()
instead of tcp_do_segment() context. Restore the expected behaviour.

Note: This call to soisconnected() seems to be extraneous in all
cases (with or without accept_filter). Will be addressed in a
separate commit.

PR: 212920
Reported by: Alexey
Tested by: Alexey, jch
Sponsored by: Verisign, Inc.