tcp_hpts: let tcp_hpts_init() set a random CPU only once

After d2ef52ef3dee the tcp_hpts_init() function can be called multiple
times on a tcpcb if it is switched there and back between two TCP stacks.
First, this makes existing assertion in tcp_hpts_init() incorrect. Second,
it creates possibility to change a randomly set t_hpts_cpu to a different
random value, while a tcpcb is already in the HPTS wheel, triggering other
assertions later in tcp_hptsi().

The best approach here would be to work on the stacks to really clear a
tcpcb out of HPTS wheel in tfb_tcp_fb_fini, draining the IHPTS_MOVING
state. But that's pretty intrusive change, so let's just get back to the
old logic (pre d2ef52ef3dee) where t_hpts_cpu was set to a random value
only once in a CPU lifetime and a newly switched stack inherits t_hpts_cpu
from the previous stack.

Reviewed by: rrs, tuexen
Differential Revision: https://reviews.freebsd.org/D42946
Reported-by: syzbot+fab29fe1ab089c52998d@syzkaller.appspotmail.com
Reported-by: syzbot+ca5f2aa0fda15dcfe6d7@syzkaller.appspotmail.com
Fixes: 2b3a77467dd3d74a7170f279fb25f9736b46ef8a

(cherry picked from commit 3f46be6acadd5d660acde67d9d4c80137f424b70)