ktls: Write-lock the INP when changing a transmit TLS session.

The TCP rate pacing code relies on being able to read this pointer
safely while holding an INP lock. The initial TLS session pointer is
set while holding the write lock already.

Reviewed by: gallatin, hselasky
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D34086

(cherry picked from commit cd0525f615fe8fea750d2078b61026aabe0d0471)