tcp: improve failure handling in tcp_newtcpcb()

In case of a failure of tcp_newtcpcb, where NULL is returned,

• call CC_ALGO(tp)->cb_destroy, after CC_ALGO(tp)->cb_init was called.
• call khelp_destroy_osd(), after khelp_init_osd() was called.

Reviewed by: glebius, rscheff
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D45753

(cherry picked from commit 14fee5324a9b76fff6e8aad266600eefab3ef335)