sysctl net.inet.tcp.ktlslist: allow snd_tag_status_str() to sleep

For this, unlock inp around the calls, taking the reference on it. If
the inp appears to be freed or unlinked after the relock, return
EDEADLK.

Reviewed by: glebius, markj
Sponsored by: Nvidia networking
Differential revision: https://reviews.freebsd.org/D51143